Atmel tinyAVR microcontroller support (ATtiny45/84/85) for the Arduino IDE (1.6.4+) using the Micronucleus bootloader.
NOTE: tinyAVR for Arduino has been tested on Windows but Linux and OSX are still under development. If using Linux or OSX, you will have to compile the Micronucleus++ uploader yourself as it is not currently included in the download for those operating systems.
In the Arduino IDE's Preferences dialog, add the following URL to the Additional Boards Manager URLs:
https://github.com/mjbcopland/tinyAVR-for-Arduino/releases/download/1.0/package_mjbcopland_tinyavr_index.json
Then install the "Atmel tinyAVR" package from the Boards Manager.
After installing, ATtiny45/84/85 options will appear in the Tools→Boards menu.
When first connecting a Micronucleus device, you will likely be asked to install the Micronucleus drivers. If you already have these drivers installed, the installer should update them. These drivers can be installed manually using the Zadig installer.
By default, most Linux distributions will not allow communication with unknown USB devices. To fix this, copy 49-micronucleus.rules to /etc/udev/rules.d/.
Burning the Micronucleus bootloader is done in the same manner as other Arduino bootloaders. See here for details.
After burning the Micronucleus bootloader, sketches can be uploaded via USB like any other Arduino sketch. You will, however, need to manually connect or otherwise reset the microcontroller when prompted.