I'm not sure if Blockland is well know here, but it is the game that led me to powder toy. Blockland is an open world sandbox where you have a selection of brick to choose from, you can build buildings, landscape and many things using these. It also has multiplayer. Website Fourms
Be warned, The community in Blockland is generally un-friendly and usually demands common sense, so bad grammar and spelling is looked down upon. It also consists mainly of 13-15 year olds, so it can get bad at times.
I believe that is ROBLOX for 15 year olds. And even now, ROBLOX has people up to ages 18 playing and liking it because it's pretty much a way to test your abilities to script and build.
I use the old alpha version of blockland (Return to blockland/Blockland V 00002 or something), it's free but the community is small after retail blockland was launched. http://s3.zetaboards.com/AndoRTB/index/ That's the link for the unofficial forum created by the few remaining old players, most are quite friendly but some are... Yeah.
I've gotten bored of it after playing it for so many years though.
It's okay, the game gets boring after a while because there is absolutely nobody that is like-able. The children are immature and seem to love memes, the teenagers are egotistical and think they know everything, the adults are like overgrown teenagers.