Pay-What-You-Want sales are arguably 'the next big thing' for indie developers this year, with 2D Boy starting it all off (or so I believe). It's probably one of the best new trends ever, so embracing as many as possible is the way forward. Caffeine Monster Software have now put their two year old Smiley's Shooty Adventure in such a sale.

It's a very trippy arena shooter for PC, with pixelated graphics and exploding squares.
It's usually $10, but until May 12th the game costs whatever you want.