every year 1up.com has a list of free games to play online and some of them can be quite fun. here is this years list:
http://www.1up.com/do/feature?cId=3177782

heres one:
Megaman vs. Metroid & Megaman vs. Ghosts&Goblins
http://www.newgrounds.com/portal/view/124070 & http://www.newgrounds.com/portal/view/175060

Quote:

Does what it says on the tin: Megaman faces off against his contemporaries, but not before navigating some really well put together levels representing their respective games. Even better, he can steal their weapon to use against the other. Play it before the cease-and-desist hits!