And now for a game that I've paid for about five times over, thanks to the Humble Indie Bundle. It's also one of the few games in one of them that I had played before those bundles became a thing.
World of Goo is a 2D physics simulator. Gameplay consists of building structures to get to another point. Oftentimes, it's building a tower to reach a pipe, but each level has some kind of novelty. Structures are built from "Goo Balls", who are the inhabitants of The World of Goo. The strategy of a level is diverse: build a tower, build a bridge, build a tower to flip into a bridge, or sometimes, just float.
There are several species of goo. Black goos are ordinary, and only useful for building permanent structures. Ivy goos are like black ones, but they can be reused infinitely. Balloon goos are, well... balloons, and lift things. Beauty goos aren't useful, but escorting them is many level's objective. Matchstick goos pass along fire when lit.
The art style is quite slick, and the palette conveys the feel of the level well. The music is rather whimical, and the creator has released it for free. There are five chapters in the game, containing about a dozen levels each. Each one has some sort of novelty, and the whole thing progresses along a weak story. At some point, the world becomes 3D, and you are thrown inside a 70s era mainframe computer, where things are and look very different.
The game is pretty short, like Portal short. You could probably finish it in about 5 hours. I recommend this to anyone who likes strategy or problem solving games.