So I made a little adapter so I can connect PS/2 mice to the pre-ADB Classic Macs (128k, 512k, 512ke, probably Lisa). They use a really simple scheme: two pairs of quadrature inputs (one for X, one for Y), and one input for the single button. 5v is provided for the mouse. Perfect for a little microcontroller like the ATtiny to interface to (which is funny, because the ATtiny is capable of a far faster clock speed than the Macs...).
There isn't much to it, just an ATtiny84 and a couple connectors. The software is actually compatible with most AVRs, so you can use whatever AVR you have handy if you're willing to rework the PCB (they just need enough pins: 7 at bare minimum if you get rid of the status LED). You'll notice a USB plug in there. This thing does not speak USB, only mouse PS2. Thing is, most USB mice can act as PS2 devices (those little PS2->USB adapters are just passive wires). So instead of fiddling with adapters, I opted to directly use a USB plug. Beware though that some really modern mice don't speak PS2. If your mouse is worth less than $5, you're set ;).
I've made all the code/schematics/PCB layouts available as open-source hardware (TAPR Open Hardware License, www.tapr.org/OHL), so grab your copy here.