Avoision
Falling Block Game 2
Avoision is a simple collect-dots-avoid-blocks game.
Falling Block Game 2 is a Tetris clone.
Both are simple games. Both are very polished! I avoided them at first, assuming they would be of the yet-another-hello-world-game type but luckily got over that arrogant moment. :)
Neither forces full-screen, they scale to any resolution, controls are crystal clear and feedback is great.
Both games use Radius Engine, which is a Win/Lin OpenGL/SDL/Lua engine. Kind of like LÖVE.
For Arch Linux users: I made AUR packages for Radius Engine, Avoision and FBG2. I use clyde for managing packages.
PS: Sometimes polish is worth more than complexity. Can you name free open source games that are polished to the max?