This is a general catch-all collection for any tilesets for side-scrolling/fighting/run 'n gun type games.  Please post a link in the comments if you see anything I've missed or should add, or if you want to be a collaborator.

NOTE:  I also maintain a collection of sidescroller/platformer characxters and enemies, which might also be of interest: http://opengameart.org/content/platformersidescroller-characters-and-enemies