Behringer UCA202 USB Interface:
RCA in/out, optical out, monitors/headphone out, volume control for monitors.
for $35 shipped.
Griffin iMic2 USB Audio Interface:
in/out 3.5mm stereo mic/line, toggle switch for mic/line setting.
for $31 shipped
problem with the Behringer one, is that I'd have to carry an extra RCA->3.5mm adaptor, if I'm going to be carrying that around. But optical out would be awesome. And volume control for the monitor out? I guess.. it'd be more useful for at home, tape->CD conversion things...