I've developed or participated in a lot of games that were under a CC Share-alike or similar license.
I'm collecting them all at a Github repository: http://github.com/jennibee/image
So far, there's sprites, music, sound effects, and backgrounds. I'll be adding more during the duration of the Dream On Contest.
Feel free to use anything else I've created at http://github.com/jennibee as well.
Also, if you'd like me to create anything specific (sprites, backgrounds, music, sound) for your project, I'm also going to be doing that this year to hopefully ease the difficulty curve of entering. E-mail me at firstname.lastname@example.org if you'd like me to make something for you.
If anyone else has any assets they'd like to allow people to use for this contest, feel free to share it in this thread as well. :)