http://2dgameartforprogrammers.blogspot.com/ - Great blog for getting some sprites and also learning how to make sprites that even a programmer can do.
http://opengameart.org/ - Another good resource for some 2d sprites
http://www.lostgarden.com/ - Huge amount of art here though kinda hard to find.
http://pousse.rapiere.free.fr/tome/ - some tile art
http://www.spiralgraphics.biz/packs/ - some textures (Pretty good quality)
http://love2d.org/wiki/Free_Game_Resources - Post that helps group some good sites together in one place
http://letsmakegames.org/resources/art-assets-for-game-developers/ - Another site that groups sites together to make art and other resources easier to find.
http://hasgraphics.com/category/tilesets/ - Less known but has some decent stuff here.
http://gameartpartners.com/downloads/category/game-art-bundles/featured-bundles/ - One of the cheaper paid art sites out there (Know you want free stuff but even so thought it might be worth posting).
There is actually quite a bit more out there so won't list it all. Though I will let you know now that scrounging the web for free art will work just for placeholders in your game while making it but you are going to eventually need a dedicated artist or will have to pay for art if you want anything polished.
The reason why is because most free art isn't of that great of quality but the even bigger concern is it is extremely hard to find free art for a whole game that matches the general theme. Every free one you get will have a different "feel" to it and will make your game art look like it is all over the place.
I would say your best bet is just to grab some free ones to use as placeholders while developing your game (Hell I even just use squares and circles sometimes) and once you get farther into the project start advertising that you are looking for a artist to help out. They will be more likely to jump on board since you have a working game (Though not polished) and just need the art.
Artists are just like programmers in that sense, they don't want to join a project just to have it fall apart and they did all the art for nothing.
Anyways wish you the best of luck with your game and hope all goes well.