Blockland, is a semi-noncompetitive multiplayer computer game built on the Torque Game Engine, in which players build with interconnecting LEGO-type blocks in virtual online worlds. Users construct massive buildings and artwork across numerous servers and then systematically destroy and rebuild them again. The game is neither endorsed by, nor affiliated with, the LEGO brand. It was spotlighted on The Screen Savers February 11, 2005, drastically increasing the user base overnight.
Blockland is available for $19.95, a demo is also available. The demo restricts access to online play and limits construction to 150 bricks. You don't need to download other things once you have entered a key.
The game is currently available on Windows and Macintosh, and will run on Linux if you use Wine.
This question was asked on the wiki: Blockland Wiki.