World Of Goo...sounds like a rediculous game name right? Well, in a sense it is, especially since the premise is building bizzare structures with little "goo balls". The game was engineered by an Indie company called 2D Boy and very much has the same feel and strife found in the classic game "Lemmings". You have a set number of goo balls, you then try to build your way to set point in order to "rescue" the little balls of goo, along the way you'r prone to becoming impaled, falling to your doom and no doubt more perile.

It's mostly based upon physics and architecture, with a splash of chaos and a speck of Tim Burton (in a sense). The graphics are relatively simplistic, the music is rather quirky as though you were in a twisted theme park but, it's the challenge of positioning just the right amount of support points and guessing the balance of the structure that makes this game so addictive. After the first four or five levels you start to get a decent feel for how building in this world works, it becomes easier to anticipate what sort of support you need to build upwards as well as outwards.

I just beat Chapter 1 (of 4, possibly 5 if the Epilogue is more than just an end cinematic). I very much recommend this little gem, taking up only 74mb of HD space, and having a very user friendly interface, it makes for an entertaining game to just jump into...quitting is the hard part.

http://2dboy.com/games.php World Of Goo demo available on their site... though, if you know where to look you can find the full version else where.

Played this a while back and it's quite an interesting game. The concept/idea is quite unique which makes for a great time-killer.

I bought it on Steam (I think it was for less than $5, a promo) and although I'm not playing it regularly I do enjoy it a lot.
2D Boy, the developper, is trying new ways of making people buy the game instead of downloading it. So watch out for Steam promos or other discount (last month, to celebrate the 1st year of the game, you could buy it for the price you wanted. Any price!), it's worth it.

EDIT: and it plays on Mac and Linux too

I have gone through 3 chapters so far. Haven't got the time to finish the game yet.
It's quite an interesting and very original game. Not hard, but a great time killer, indeed.