I Wanna Be The Guy is a few years old, but looks like a very entertaining retro style indy game. I haven't played it yet, but from videos and reading about it the game seems to be pretty difficult. Looks like fun, and free to download.
A great source of info for the Famicom system (Japanese NES). There were so many great games that never made it to the US for the Famicom, and many games that did make it suffered from editing or changes. A couple Famicom highlights: Gryzor (Contra with a couple cool graphics added), Akumajou Densetsu (Castlevania 3 with many cool differences), FF2, FF3, Crisis Force, Moai Kun, Parodius, SMB2J... So many great games for the retro or NES fan!
I Wanna Be The Guy - The hardest damn game ever, probably because it is designed to be unfair. That said, it is quite clever in its use of famous video games and mechanics. Just watch a video of someone else playing it.
Four Guys Trying To Play Mario With Portals Is Funnier Than It Has Any Right To Be