Something with a big enough tank so the compressor itself isn't running all the time when using the air tools. Also, if you are planning on a lot of grinding or cutoff wheel use (continuous duty tools) get a compressor with a high CFM capacity... if you are planning on air ratchet or air impact gun only you can go with a lower CFM capacity compressor.
I found a Coleman compressor at Costco - 30 gallon or so for a good price. I'd suggest investing in a better one than that, as I plan on doing at some point. Doesn't need to be much bigger than that though. I'll give some specs on mine tomorrow.