This amazing project was started way back in 2013 by veteran PixelJoint member Hapiel.
I was actually also a member of PixelJoint at the time and always remember Hapiel as the black cat wearing a yellow shirt and a red cap ( his avatar at the time).
The project made use of DawnBringer32's awesome 32 color palette to give it a consistent color scheme.
While early contributions fell under the CC-BY-3.0 license, the latest iteration is under the public domain and completely free!
It is currently moderated by Damien, Gabriel and Hapiel.
They explain it better when speaking about their project so be sure to visit the about and history sections on their site.
Also be sure to check out the list of contributors ( they are all fantastic pixel artists ).
Since I am a robot guy, these are my favorite sprites from the project:
OPP stands for the Open Pixel Project, and is a collaborative effort to create freely available art assets for games, shared in the public domain! That means anyone is free to edit, rip, steal, use, share or do whatever you wish with these assets in any way, without any obligations to us.