Do you have some old DVDs that you no longer watch? Want to trade them for some others that you might like better? Try this service called SwapaDVD!
You post 10 DVDs (or more) that you're willing to trade for other DVDs. For posting your first ten you get one free credit. You cash in your credits for DVDs that you want.
We have been using this website for awhile now, and it works pretty well. We have received five DVDs and have credits for six more. Our Clifford and Bob the Builder DVDs were the first to go, so if you have DVDs your kids have outgrown, this might be a good way to dispose of them.
It costs about $1.85 to mail a DVD, which is often cheaper than renting a movie these days! The DVDs are used, but so far I've only had to request one credit refund because the DVD was smashed in transit. I got the credit immediately and got another DVD a few days later. It's not a perfect system, but it has allowed me to get some DVDs that I would never have purchased for full price.
There is also a similar service for books, which I also use. I have found that the books I want aren't as readily avaiable on this site as I'd like, though they have millions more than the DVD service. Also, the books I have to trade haven't been as popular as my DVDs either. Still, when we lost a library book this summer, I was able to find it at PaperbackSwap instead of having to pay the full new price. (Unfortunately, we found the library book two days after I got the replacement book, so now we have a copy of 101 Dalmations of our very own. Sigh.)
There is a CD swap site also, but I they charge a small fee for the CDs instead of making it a straight-across swap. I haven't used it and can't offer any feedback.