World of Goo was developed by independent game writers 2D Boy ("I love you 2D Boy!"), using lots of open source components brought together to make a delightful little game.
The story is really a backdrop to the gameplay, which consists of taking little goo balls and assembling goo-ey structures like towers and bridges that oscillate and undulate under the cumulative weight of the goo balls you attach onto them. Attach too many, and it will buckle. Attach too few, and you won't be able to make it to your target.
Every level has a start and an end point, and you're given a certain number of goo balls to assemble a structure to get there. You're supposed to have a few goo balls as surplus at the end of it, and the challenge is to have enough goo balls free, i.e. not used as part of the structure, to win the level. They get sucked up into a big vacuum machine and put away.
The story has something to do with the "Goo Corporation" and some corporate conspiracy. The goo balls are self-aware and extremely adorable, but not aware that they were designed by the Goo Corporation, are used in "industry", and that they are incredibly tasty.
The core of this game is its physics engine, which was used from an open source software library. The goo-ey structures oscillate and undulate under the weight of the additional goo balls you attach onto them, and obey the laws of physics much how you would expect them to, (with sufficient suspension of disbelief). The engine seems to have integrated variables of mass, center of mass, moment of inertia, friction, and joint strength (something I never studied in high school enough to name), where structures that are too heavy buckle under the weight and become mere attachments instead of rigid support.
A host of other open technologies were used to assemble the game, and the core development team was extremely small. But they have done a superb job of integrating these technologies into a cohesive, wholesome product that is fun to use, play, and share.
Re-playability and Value
The game is tremendously replayable. Once you've finished all the levels you can go back and play them and see how you could solve the levels differently, assembling the goo balls in weird ways to achieve your goal.
Also you can play the game under "OCD" rules. Obsessive-Compulsive Disorder sets a higher standard for you to finish the levels, with almost double the number of surplus goo balls you need at the end of the final structure to qualify as having finished it under "OCD" rules. You have to really maximize every goo ball you use, pushing the structure to its very limits and depending on environmental factors to maximize your use of goo balls. Some real "out-of-the-box" thinking goes into this!
You can also upload your scores and structures to the web, and share with others. And all the surplus goo balls you collect at the end of every level is accumulated in a very large, open level where you can build pretty much anything you like. A "carte blanche" for your goo-building needs. The highest point of your structure is represented by a small whiff of cloud with your name and country on it. It bobs up and down, and rises and falls as your structure goes higher and higher, and you can compare with others.
The only very minor downside I found to this game was the menu system, before the actual game starts. It seems a bit neglected and rushed, with the bulk of developer attention going into the gameplay mechanics, no doubt. This product was delayed by almost 6 months (their interview with X-Play had them naming a release date at around Jun. 2008, and it was only released in Dec. 2008). I'm not complaining, though, because the game works where it counts: in the game. This is just a nitpick.
I'd give this game 5 stars out of 5, if I had stars to give. It costs only 20 USD, making it very cheap for the hours of fun you'll have with it.
My Ideological Pitch
I encourage people to support independent game developers, people writing great code in their backyards, people with common sense and will power to use computers to make good, wholesome magic. In a world of re-hashed first-person shooters with gratuitous guts, gore and ample bosom for pimple-faced teenage males, this is a very welcome change. We should support a good product by buying it, especially if it's made by "the little guy." I voted with my dollar on this one, and you should too!