Monster Island is a set of tabletop rules for giant monster combat. Grab some toy monster figures off the shelf, use the monster creation rules to design your monster of choice, and then fight it out for supremacy.
This game is very well done and is a great example of the advantages of PDF publishing for niche game products. There are numerous expansions and supplements on the Monster Island site that take the game in pretty much every direction a giant monster game should go — robots, human armies, alien invaders, etc.
Watch out, Tokyo Tower, here we come.