World Of Goo is now out (for pre-orders, that is. Actual game comes out in a week).
http://2dboy.com/
And I have to say it is one of the best games I have played in a very, very long time. The puzzles are both hard and fun to solve (no head-ache causers) and I'm having a blast replaying it.
The difficulty increases throughout the game, but it not by raising the conditions or adding a timer. The puzzles themselves become harder to solve requiring more thought to them.
The music in the game is amazing. I've added some of the tracks to my ipod even, which is a first for me about game music.
I laughed out loud several times at the puzzles and their solutions; not because they were funny but because they were so genious. I don't want to spoil anything, but many of the puzzles will leave memories.
I really, really recommend everyone checking this game out. The wii will be getting it (download only) and will have multiplayer. The game (PC) has no drm on it =)