The Arduino Component Sketches are a library of ATTiny sketches that can be used to create small synth building blocks. I have used them mostly in bread board circuits. Think of it like a 555 but with much more functionality.
- Rad-fi System from Bleep Labs Uses Arduinos as building blocks for Synths on a breadboard. A collection of ATTiny micros with these sketches would be completely compatible with this system.
- Bastl Kastle uses 2 ATTiny85s that can be programmed with other features.
- Electric Druid has some AVRs and others as building blocks. For example, an LFO.