Fast Track Pro is indeed a class compliant device so no driver install is absolutely necessary, but installing the driver allows for input/output configuration via System Preferences pane plus I saw a decent performance improvement in latency and CPU usage.
Behringer UCA222 (USB audio) - Working fine for me.
Costs around £22 here in the UK so is a cheap option if you just need a simple RCA 2 In/2 Out based setup. You cant plug in 1/4 inch jacks (eg. guitar leads) or XLR plugs but it does have an optical out!
Confirmed Pro Audio Interfaces & Drivers - Mac OS X
The StealthPedal USB interface from IK Multimedia works perfectly as an audio interface and virtual pedal controller on Mountain Lion (I'm sure it works with the other OS cats too). Also, if you need to use it as a MIDI controller, it does that too! It does not require any manual installation of drivers; it is virtually plug and play!