Available as Arduino Board Manager entry "Digistump AVR Boards" using the Board Manager URL: https://raw.githubusercontent.com/ArminJo/DigistumpArduino/master/package_digistump_index.json

New micronucleus bootloader and binary version 2.6 with info function.
First version without MH-ET LIVE Tiny88(16.0MHz) support, since it is now supported by ATTinyCore.

Another great core for ATtinies with ongoing support is the ATTinyCore of Spence Konde, which currently added micronucleus support. Use the Board Manager URL: http://drazzy.com/package_drazzy.com_index.json for it.

• To shrink generated code size by 5 to 15 percent (depending on your code) the lto flag was added in platform.txt. To help finding code which consumes the flash, the generating of disassembler and memory map files are added. For Arduino IDE you will find these files in C:\Users<Name>\AppData\Local\Temp\arduino_build_.
• To reflect the smaller bootloader size of micronucleus version 2.5 which allow 6586 instead of 6012 bytes (+11%) the upload.maximum_size entry was changed in boards.txt.
• Since the original gcc version does not support the lto flag, the compiler path was changed to the latest Arduino gcc (7.3.0-atmel3.6.1-arduino5) in the package_digistump_index.json.

• Added #define LED_BUILTIN in some pins_arduino.h.
• Added #define __FlashStringHelper fstr_t in Print.h.
• Updated broken extensa links for xtensa-lx106-elf-gcc in package_digistump_index.json.
• Included most pull requests done after the 1.6.7 release like recipe.output.tmp_file={build.project_name}.hex in platform.txt and EEPROM library.
• Added recipe to update booltloader by Arduino IDE.
• Removed the non trusted post_install.bat since Arduino code states: // Set main and bundled indexes as trusted => all others are untrusted.
• Extended DigisparkKeyboard library with 22 keyboard layouts from Teensyduino Core Library, fixed bugs, documented and improved it. Here it is documented how the mapping works, so now you can fix wrong mappings by your own (but please give me feedback to include the fix).
• Fixed and extended a growing number of examples.

To get all the benefits, just replace the old Digispark board URL http://digistump.com/package_digistump_index.json (e.g. in Arduino File/Preferences) by the new https://raw.githubusercontent.com/ArminJo/DigistumpArduino/master/package_digistump_index.json and install the Digistump AVR Boards version 1.7.2 or later.

1. Download this repo.
2. Create a directory C:\Users\AppData\Local\Arduino15\packages\digistump\hardware\avr\1.7.x and extract the digistump-avr-1.7.x.zip to it.
3. Create a directory C:\Users\AppData\Local\Arduino15\packages\digistump\tools\micronucleus\2.5 and extract the tools\micronucleus-cli-2.5-i686-mingw32.zip to it.
4. Extract the file package_digistump_index.json to *C:\Users\AppData\Local\Arduino15*.

As reference you may look at your C:\Users\AppData\Local\Arduino\packages directory to see how other packages are installed.

For Windows you must install the Digispark driver before you can program the board.
if you have the Digistump AVR Boards already installed, then the driver is located in %UserProfile%\AppData\Local\Arduino15\packages\digistump\tools\micronucleus\2.5\Digistump_Drivers. Just execute the Install_Digistump_Drivers.bat file.
Or download it here, open it and run InstallDrivers.exe.

Different bootloader configurations are available here.
To update your old, flash consuming bootloader, you have 2 choices.

1. Use the new Digistump board manager URL (see above), choose a bootloader with Tools > Micronucleus variant and then burn it with Tools > Burn Bootloader.
   
   

2. Run one of the Windows scripts like e.g. the 0_Upgrade-t85_recommended.cmd. The internal mechanism is described here.
   You may also test the t85_agressive bootloader, a configuration overwiew is available here. It works for my boards but the USB timing is not guaranteed as stable as in the other configurations.

The meaning of fuses can be seen with the Engbedded Atmel AVR® Fuse Calculator. Windows helper scripts for setting fuses can be found here.

The default fuses for a Digispark board are:

• ATtiny85 Lfuse: 0xE1 - (digispark default) PLL Clock + Startup 64 ms
• ATtiny85 Hfuse: 0xDD - External Reset pin enabled (Pin5 not usable as I/O) + BOD 2.7 V + Enable Serial Program and Data Downloading
• ATtiny85 Efuse: 0xFE - self programming enabled.

BOD enabled requires additional 20 µA in sleep state and therefore may be not desirable for low power battery applications. To disable BOD, use 0xDF as Hfuse.

The default fuses for a Digispark Pro board are:

• ATtiny167 Lfuse: 0xFF - External crystal osc. Frequency 8-16 MHz + Startup 65 ms
• ATtiny167 Hfuse: 0xDC - External Reset pin enabled + BOD 4.3Volt + Enable Serial Program and Data Downloading
• ATtiny167 Efuse: 0xFE - self programming enabled.

This can only be done by means of a High Voltage programmer.

You require 16.5 MHz only if your appliction uses the V-USB library, i.e. if it acts as an USB devive like mouse, keyboard etc.
In all other cases it is recommended to use the standard CPU frequencies of 16, 8, 1 MHz. This is the frequency the CPU is running after the bootloader part, which must run with 16.5 MHz!

Information about reducing power consumption can be found here and here.

                       +-\/-+
 RESET/ADC0 (D5) PB5  1|    |8  VCC
  USB- ADC3 (D3) PB3  2|    |7  PB2 (D2) INT0/ADC1 - default TX Debug output for ATtinySerialOut
  USB+ ADC2 (D4) PB4  3|    |6  PB1 (D1) MISO/DO/AIN1/OC0B/OC1A/PCINT1 - (Digispark) LED
                 GND  4|    |5  PB0 (D0) OC0A/AIN0
                       +----+
  USB+ and USB- are each connected to a 3.3 volt Zener to GND and with a 68 Ohm series resistor to the ATtiny pin.
  On boards with a micro USB connector, the series resistor is 22 Ohm instead of 68 Ohm. 
  USB- has a 1.5k pullup resistor to indicate a low-speed device.
  USB+ and USB- are each terminated on the host side with 15k to 25k pull-down resistors.

Digital Pin numbers in parenthesis are for ATTinyCore library

                  +-\/-+
RX   6 (D0) PA0  1|    |20  PB0 (D8)  0 OC1AU
TX   7 (D1) PA1  2|    |19  PB1 (D9)  1 OC1BU - (Digispark) LED
     8 (D2) PA2  3|    |18  PB2 (D10) 2 OC1AV
INT1 9 (D3) PA3  4|    |17  PB3 (D11) 4 OC1BV USB-
           AVCC  5|    |16  GND
           AGND  6|    |15  VCC
    10 (D4) PA4  7|    |14  PB4 (D12)   XTAL1
    11 (D5) PA5  8|    |13  PB5 (D13)   XTAL2
    12 (D6) PA6  9|    |12  PB6 (D14) 3 INT0  USB+
     5 (D7) PA7 10|    |11  PB7 (D15)   RESET
                  +----+
  USB+ and USB- are each connected to a 3.3 volt Zener to GND and with a 51 Ohm series resistor to the ATtiny pin.
  USB- has a 1k5 pullup resistor to indicate a low-speed device.
  USB+ and USB- are each terminated on the host side with 15k to 25k pull-down resistors.

Digital Pin numbers in parenthesis are for ATTinyCore library

                       USB
                     +-\__/-+
              PA2  15|      |14  PB7
              PA3  16|      |13  PB5 SCK
           17 PA0  A6|      |12  PB4 MISO
           18 PA1  A7|      |11  PB3 MOSI
     (D17) 19 PC0  A0|      |10  PB2 OC1B/PWM SS
     (D18) 20 PC1  A1|      |9   PB1 OC1A/PWM
     (D19) 21 PC2  A2|      |8   PB0
     (D20) 22 PC3  A3|      |7   PD7 RX
SDA  (D21) 23 PC4  A4|      |6   PD6 TX
SCL  (D22) 24 PC5  A5|      |5   PD5
     (D23)    PC1  25|      |4   PD4
RESET         PC6 RST|      |3   PD3 INT1
LED           PD0   0|      |5V
USB-          PD1   1|      |GND
USB+  INT0    PD2   2|      |VIN
                     +------+
  USB+ and USB- are each connected to a 3.3 volt Zener to GND and with a 68 Ohm series resistor to the ATtiny pin.
  USB- has a 1k0 pullup resistor to indicate a low-speed device.
  USB+ and USB- are each terminated on the host side with 15k to 25k pull-down resistors.

  Pins not available: PB6 is CLOCK_IN, PC7 is 25

The Arduino ESP8266 core available with https://arduino.esp8266.com/stable/package_esp8266com_index.json supports the Digistump Oak board now, better use that.

• New micronucleus bootloader and binary version 2.6 with info function.
• First version without MH-ET LIVE Tiny88(16.0MHz) support, since it is now supported by ATTinyCore.

• Defined symbols DIGISTUMPCORE, DIGISTUMPCORE_MAJOR etc.
• Last version with MH-ET LIVE Tiny88(16.0MHz) support, since it is now supported by ATTinyCore.

• Improved bootloaders.
• Added Attiny support to usbconfig.h.
• Added Burn Bootloader Method like in ATTinyCore.
• Removed digistump-sam from repo.
• ISR_NOBLOCK in millis interrupt only active for 16.5 MHz setting.
• Improved DigiKeboard.h and keylayouts.h.

• Updated examples and added warnings for CDC examples.
• Version to force update of all 1.7.1 changes.

• New micronucleus binaries for all platforms.
• Bootloaders with updated MCUSR handling.

• Support of the cheap Chinese MH-ET LIVE Tiny88(16.0MHz) board with ATTinyCore.
• Corrected wrong mouse buttons PR#2.
• Updated boards.txt to make use of menus. Therefore some board names changed like digispark-tiny1 -> digispark-tiny:clock=clock1 etc.
• Set OSCCAL to factory default instead of using a rule of thumb, which was not reliable.
• Added linker flag -mrelax. This reduces code size at least for ATtiny167.

• The original Digistump version with lto and changed upload.maximum_size for micronucleus bootloader 2.5.
• Added LED_BUILTIN definition in pins_arduino.h.
• Added #define __FlashStringHelper fstr_t in tiny/Print.h.
• Updated broken extensa links for xtensa-lx106-elf-gcc in package_digistump_index.json.
• Included most pull requests done after the 1.6.7 release like recipe.output.tmp_file={build.project_name}.hex in platform.txt and EEPROM library.
• Added recipe to update bootloader by Arduino IDE.
• Changed message for BIN redefined in Print.h and it can be suppressed by commenting out #define SUPPRESS_BIN_WARNING in line 39 in Print.h.
• Commented out the annoying #define true 0x1 and #define false 0x0 in wiring.h.
• Removed unnecessary files like .gitignore.
• Added #define TwoWire USI_TWI in Wire.h.
• Changed #include <avr/delay.h> to #include <util/delay.h> in 3 files.
• Included Multi-Keyboard Layout MOD.
• Redefining BIN to 2 for print. BIN can not longer be used as bit position (7) for the x5 ADC "Bipolar Input Mode".
• Enabled all warnings at compile time and removed compiler warnings in tiny core.
• Modified definition for yield() in WProgram.h to avoid error "undefined reference to `yield()'".
• Enabled compiler.cpp.extra_flags.

Please write me a PM including your motivation/problem if you need a modification or an extension.