So theres this awesome thing I just recently found called garry's mod.
Its basically a giant sandbox mode for the source engine (what valve uses for its game now, ie. Half Life 2, Portal, Team Fortress 2)
You can spawn models from any source game you own, spawn some npcs, vechicles, make structures, do a bunch of complicated wire stuff, and get addons and mods to make it even better.
Examples of awesome:
Dropping a Star Destroyer on an army of G-mans
Take a Heavy from Team Fortress 2 and freeze his model in fun positions (you get a face edit, can screw with ragdolls, etc.)
So you can see more at
http://www.garrysmod.comRight now it costs $10 off steam, as opposed to the last 9 versions that were free, but from now on there will just be big updates.
Oh, did I mention ONLINE MULTIPLAYER?!?!