As noted above, the rear derailer (per Sheldon Brown, or derailleur) doesn't much matter as long as it can handle the capacity you need and matches the shifters (i.e., there may be problems using SRAM shifters and Shimano derailers, or vice versa). The Shimano Alivio M410 Rear Derailleur ($29) would work fine, or if you prefer one step up on the Shimano marketing scale, the Shimano LX M581 Rear Derailleur ($49) is capable too.

This may be old, but could be useful:

http://www.wheelworks.co.nz/drivetrain.php

If you use friction shifters, or indexed shifters in friction mode, most any rear or front derailer will work as long as it can handle the capacity.