NeoCracker winning this thread so far, for recommending something I hadn't heard of before.

Startropics for NES is brilliant. It's sequel (Zoda's Revenge) is also brilliant. You should totally legally hunt down copies and not just find ROMs and play them because it's free and easy... I've also been playing the crap out of Costume Quest and Brutal Legend, and diving into CaveStory on occasion, all recommended.

bleh overdone shoutout to Fallout because everybody already knows it's both awesome and glitchtastic.

Oh, and all fighting games in the last couple years. Seriously, I know a lot of people who have refused to play fighting games since the sega/SNES days, but the retro-comeback has been obscene. Street Fighter IV in any form, the Mortal Kombat reboot, and as of today I can vouch for the gameplay Street Fighter x Tekken. Virtua Fighter 5 as well, but then again Virtua Fighter has always been awesome. They're simple again, they're fun again, so go play some of them.