If you love Legos, but hate stepping on them in the middle of the night, you need Lego Digital Designer.http://ldd.lego.com/
Think of it as a CAD system for Lego freaks. It comes complete with a catalog of every brick they make, in all possible colors. Build the model of your dreams, and it'll even print out a parts list so you can build it in real life. Or, use their online ordering wizard and have everything you need shipped straight to you.
The whole thing is free, unless you decide to jump online and buy enough bricks to recreate key scenes from The Matrix trilogy.
The new version adds Mindstorm support and a cleaner interface, but I can't vouch for that personally. My little girl has been using the program since it first came out and actually prefers the virtual legos to her real bricks.
Which is great, since turning off the computer is soooo much easier than picking up the floor.(Image ganked from Softpedia.)
UPDATE: Forgot the durn link. D'oh!