I use this site to have my students complete the "remix/revision" assignment for GWRIT 103. They have some good media available for free, plus users can upload their own media.
The participants in a class I am facilitating this semester, are all required to keep a regular blog. Jeremy Alrich has been a prolific blogger long before my class, and he pointed toward a site where you can be "simpsonized". Needless to say, I took the opportunity to check this out. I am now officially "simpsonized".
For me, my big time waster is Pro Sports Daily. This is a cool little aggregator site that collects all the newspaper articles on your favorite sports team onto one handy page.