A lot of cities, counties, and landfills have free recycling or "dump on us" days that include hazmat and electronics.
Depending on their condition, you might also consider giving them to a local charity. Just make sure you properly wipe the hard drives first with something like Boot and Nuke. My general modus operandi with old computers for the last few years has been to wipe the drive, stick a Linux flavor on them, and drop them off at the Salvation Army.
Actually make sure you wipe and/or physically destroy the drives before you send them to the dump as well, if you go that route.