It depends upon the model and condition. If they are beyond repair we send them for recycling (t-mobile provides paid recycling envelopes with every handset upgrade).
If they are in good working condition, we have sent them off to FullCircleWireless.com: Cash for Used Cell Phones | Sell Used Cell Phone
for a few bucks. Last batch we sent them we got enough money for an IT go karting trip.
Also we have donated some to a local elementary school that receives money for old cell phones.