That's actually a really cool idea. Since I'm no artist, I thought this would be a great resource for game dev but then realised how out of whack everything would look if I put the parts I need to use. I could probably edit some too so that they are more consistent, if the license allows.
Not sure how useful this is, seems like it would be pretty limiting without spending a significant amount of time customizing assets.
If your in the market for low-end art, there are offshore studios that produce decent stuff incredibly cheaply. $15-$25 hr range. Mid to high end stuff is more difficult and expensive to procure for indies since most really good video game artists have full time jobs at the larger studios and not much spare attention for freelance projects.
Ideally as an indie you want to partner with a really good well rounded artist who devotes their full time attention to the project, when that happens you get unstoppable teams like 2d boy and team meat.
I have been using this site for about a year now. Some of the art on here is fantastic but you usually need to do a little bit of work to get it just right for your game.
Also, finding good art can be tough. The search function has gotten a lot better in the last couple months though. I suggest browsing relevant collections, so you can take advantage of the work people have already done to find good stuff.
Kenney has some very good content on here. You can make a fully fledged platformer with all his stuff and it'll look great, perfect for learning. http://opengameart.org/users/kenney
IIRC this came out of the Liberated Pixel Cup. A contest to create games and artwork which is available under a permissive license, allowing you to use the artwork in your own games. Very cool.
Nitpick: non-software can't be "open source," as the "source" refers to source code. Neither does "open source" refer to libre usage terms, as it only means the source is available, not that it's licensed favorably.
The "source" for a bitmap image (PNG/BMP etc) can be a vector image (SVG, PostScript etc) or maybe a multi-layer bitmap image (Gimp, Photoshop etc). The "source" in "open source" refers to that the files that were transformed into the files in question are available under a libre license, not what kind of file format or language they are represented in.
http://opengameart.org/content/recolor-all-the-items
Uses the Dawnbringer 16 palette. This is an awesome 16 color palette that can represent a lot of things and looks great.
http://www.pixeljoint.com/forum/forum_posts.asp?TID=12795
There is also a 32 color variation:
http://wayofthepixel.net/index.php?topic=16078.0