Terry Cavanagh is one of the smartest programmers out there, he produces amazing games and the best thing is that most of them are free! I've linked to his website and although I love all of them I do have a couple of favourites:
Don't Look Back- A simple graphic design but an addictive game.
Hexagon- A super addictive game made by Terry Cavanagh at a game jam. He them polished the idea into the amazing Super Hexagon, twitch gaming at its finest!
Naya's Quest- as a fan of 'Head Over Heels' this game appealed to me!