World of Goo is a great game! It is very easy to learn. You can play a quick game when you have the time and come back to it later. It is addictive and fun. No obvious bugs. It is cheap ($20) and carries no obnoxious DRM. It does not need a powerful computer but still has attractive graphics. It is available for Mac, Windows and Linux (and Wii.)
At the price point of 15 euros it is starting to become questionable if it is even worth the time and trouble to pirate this game. For me it was easy to decide to buy it.
Get the demo, play it, then buy the full version. Support the couple of guys behind this piece of pure genius!