OpenArt.moosader.com is a personal project, created as an extension of moosader.com.
It was created as a resource for hobbyist game programmers to get decent artwork and music for their games, though the assets can be used for any purpose. All the assets on this site are free to use for both non-commercial and commercial use, and no credit is needed, though others might find it helpful if they knew where your art was from.
OpenArt is in the same spirit as Open Source software, but for artwork. The art here is not under the Creative Commons license, which I find a bit wordy and difficult to understand. I just wrote up a quick license for the art that basically says: Do whatever you want, just don't swipe the art and try to sell the unmodified works on their own (for example, selling a disk of "game artwork!" that just contains the contents of this site, without any extra contributions to the art from anyone else).