Freeware, abandonware, ROM hacks, open source- there is an interesting selection of games out there for free. What have you played? Anything so good you would have payed for it?
First thing that comes to mind for me is Studio Pixel's
Cave Story, a Metroidvania free game that was simultaneously really cute, rather dark (poor Toroko and King...), and had some kick-ass weapons. Balrog won my heart pretty effectively as well.
(I'm not sure what the forums policy is on ROMs, but surely discussion of them is fine? No links, folks.)
I also hated/loved a Zelda ROM hack called
Parallel Worlds. Fiendishly difficult and irritating, but challenging in the sort of way that made you want to beat it just to have the last laugh.
What about ya'll? What have you played and loved?