I recently purchased Portal for a dollar. I may not end up playing it, but it was a popular game at one point, and I have always been curious what it was like. A dollar to have it in my Steam library doesn't seem like too much.
Metroid: Other M, Okami, and Tales of Symphonia: Dawn of the New World.
I've had my eye on them for a long time, so I finally went ahead and bought them. I needed to expand my Wii collection, anyway.