A while back, LEGO killed the LEGO Digital Designer software, but now they have partnered with Google to bring 3D LEGO design to Chrome. It's basically Google Maps with LEGO, in 3D. Currently it is limited to building in Australia and New Zealand, but more locations will be added over time. Check it out:

http://www.buildwithchrome.com

Just tried it in Firefox, seems to work fine. Looks interesting.