With the 'Joy' method, you give a gift (which is fun), someone receives one (which is also fun), and you both share the joy of giving the computer an extended life (more fun). One way is to use the 'free' section of Craigslist - just post your equipment, and folks will email you if they want it. The gift getter usually picks up the equipment; it will usually be gone within minutes, no matter what it is. You can also try freecycle as well; their motto is 'changing the world one gift at a time'. Make sure you use your local group, there are a lot of them.
The second method is the 'Profit' method. Reselling used gear is not something people (or companies) often think about, but it can be very lucrative - this guy made 300 thousand dollars selling 10 used Cisco cards. For starters, you can try eBay's rethink program, which specializes in computer equipment, or you can try and sell it yourself on eBay or Craigslist. Reselling used equipment can take more work than you think - you need to take pictures, have a good write up, process the payments, etc. - and is better suited for expensive high-end gear, very new equipment, lighter items (because of shipping), and parts (surprisingly valuable). Since there's probably a few computers hidden in your basement, why not give the planet, others, and yourself a little treat today?