I have been having a grand old time looking through the stuff at http://opengameart.org/
, especially the public domain stuff.
I have already seen a number of things that will definitely be incorporated into T.U.C.Q., DeiMWolf, and The People's DooM (A community project IWAD I started years ago that was never finished) right away! I am especially excited by the public domain resources I can reedit or combine together to create new stuff.
I am finding very useful.
It is also a lot of fun to look through the stuff.