I was checking out the 10th annual Indie games festival to see if there was anything I fancied buying and did. Despite not even getting a demo yet as the game hasn't been released (sometime in February), I ordered World of Goo. Thankfully they do have two versions of a trial game that World of Goo is based on called Tower of Goo and Tower of Goo extended. All three games are physics construction games, though the World of Goo has much more (including a profanity patch, so that the adults amongst us can laugh at some naughty language, lol. Anyway, the tower games are free as far as I can tell, and IMO are quite fun to play for a short spells of time.
You can download both versions (I haven't figured out what the difference is yet as after playing one of them I though immediately to come here and post what I'd found. I may return to explain (if any) the difference. But as it's approaching 4am ere in the UK I doubt I will yet lol. Anyway, I've still got a few other games to check out. (I'm testing a beta version of a custom map made for the recently released Crysis game, as well as a new as yet unreleased mod called BF2 Marines for the relatively old but still massively subscribed game Battlefield 2, which is actually brilliant (the mod) as it adds load of new innovative stuff to the original game. (scuze my waffling)
Tower of Goo unlimited:
http://experimentalgameplay.com/game.php?g=17
Tower of Goo:
http://experimentalgameplay.com/game.php?g=1
Another one that I think can also be found in the 101 list that FreewareFan posted the other day is plus or minus, I remember playing it around Easter last year and thought it quite cool.
Plus or Minus:
http://experimentalgameplay.com/game.php?g=308
and in fact yet another link to a whole bunch of Physics based games (32 in total):
http://experimentalgameplay.com/show.php?mode=tagsearch&tags=physics
Bsically I found them all by following one link from the Indie Festivals link, and am already getting excited at the prospect of more games :)