www.swapadvd.com
Benefit? get rid of DVD's you do not want for ones you think you want.
Cost? - cost to mail your DVD (with case $1.89) to someone else who has requested it.
Catch? - none that I have found - have now used the service for about 2 months. At some point they will probably start charging you for the service.
Its not super fast no instant gratification - ie: you have to wait for the disks that you want to be come available - specially if they are a popular request. But you can filter and search only for disks that are already available.
They have similar services for CD's and Books and the credits can be transferred from one media to another.
1 Credit = 1 Disk: so a 2 Disk DVD is worth 2 credits.

