- HID devices
- many more... the drivers come from the operating system
- MIDI devices (input notes from sensors, or generate outputs from notes, etc)
- Custom devices (you have to write your own driver though)
- There are a few more rare applications
The easiest and most useful is an HID keyboard, which is a device that does not require a driver, and acts just like a plain keyboard, but a little more customized
I have created an Arduino library that will transform your Trinket into an USB keyboard, and give you an example of how to use the library to create a two button keyboard.