Free to download set made by some great artists. not complete but still a great start for any project.

http://opengameart.org/content/ open-pixel-platformer-tiles-sprites

The OPP project was started in april 2013 by Daniel 'Hapiel' Siegmund. Since then many contributers have submitted artwork to this set.
You can use all the art from the OPP project ANY WAY YOU LIKE, as long as you CREDIT ALL THE CONTRIBUTORS!
The OPP project artwork is released under a CC-BY-3.0 license

All artwork in this set is pixel art, limited to the 32 colors of the DB32 palette (included).
If you would like to make your own art in the OPP style, the styleguide can be found here
http://www.pixeljoint.com/forum/ forum_posts.asp?TID=16248&PID=173455#173455

Some of the artworks are unfinished, but the license allows you to modify of course.
If you are interested, browse trough the different forum threads to find some more inspiration and unfinished works that you are alowed to build on!
Have a look at the 'demo' folder, it shows how the tiles and sprites can be used. Sorry, not all of the stuff found in 'demo' can also be found in 'tiles'...

PLEASE CREDIT ALL OF THE CONTRIBUTORS IF YOU USE ANY MATERIAL FROM THE OPP PROJECT.
CONTRIBUTURS:
Daniel Siegmund (Hapiel)
Damian
Dawnbringer
Ellian
Squirrelsquid
RileyFiery
Noburo
Numberplay
Skeddles
a3um
Surt
Stava
Scarab
guima1901
Conzeit
gogglecrab
anodomani
yaomon17
Ails
Letmethink
Grimsane
Diggyspiff
PypeBros
pistachio
nickthem
Crow