I didn't see this linked anywhere and consider it an essential tool that's great for killing a "little" time at work by playing around with builds/gear/troops/etc.. Should be sticky'd somewhere in my opinion.
Hero Builder - Online Flash Prgm
I didn't create this and I don't know who did, but it is amazingly well done. The only unfortunate thing about it is that it seems you can't create individual links to unique builds.