Either put it for sale (on sedo or other place), or simply release it, so that whoever cares can grab it.
You could donate it. I've never done it myself, but here are some options:
A recent post on the Boston Ruby Group list recommended this:
https://spreadsheets.google.com/viewform?formkey=dEI2dG9HT1FQMWxpcjZtVDRQUWRsQ3c6MQ And a quick Google also brought me to this:
I like the idea of donating.
You can also sell domain names through broker websites such as sedo.com. Do your research on the service you choose first though. You may find that you have the name listed for quite a long time before it sells.
This comes up frequently on Hacker News. People have been giving domain names 'free to a good home'. You should check it out.