World Of Goo is a very fun and addictive puzzle game. It requires the player to really think about what he (or she) is doing. You take control of a small squad of goo balls; some stick to walls, some float, and some just mold. You then position them in just the right place to get you to the drain, and get a certain amount of unused goo balls through a pipe. As you continue to complete levels, you earn extra goo balls which you can use in free play mode. (aka goo factory.) It has a few hundred complex levels, and different difficulty settings. It also allows you to have up to 5 profiles, so you and your friends can all play.
In some of the harder levels of the game you will navigate your goo balls up monster's throats, across long dangerous bridges, and sometimes even into caves. The game comes with built in leader boards (internet access required for this feature).
World Of Goo has won quite a few awards for best puzzle/indie game. It doesn't even require a new PC. World Of Goo is a worth while game.