Unreal Development Kit & Unity3d: Free (as in beer)
For those of you who are interested, and because it's exciting, I thought I'd mention that the Unreal Engine 3 Development Kit is now available for free for anybody to download: UDK - Unreal Development Kit - Epic Games
Yes, the same stuff that powers Gears of War, any of Epics' games and every UE3-branded title there is. Free! This includes all of the ridiculous stuff that comes with that: real-time networking, AI, real-time lighting & shadows, Nvidia-powered PhysX, distributed computing, UnrealScript, destruction tools (so you can created destructible environments,) cinematics, animation, particle effects, bink video codec support, terrains, audio, etc. etc.
You do not get source code, but it's totally free for noncommercial use and the license is fairly agreeable for small development teams, at least up to the point you can get your own source license anyway. Windows only, and the system requirements are pretty intense.
I thought the game guys out there might like to know. :) I think there's a lot of room for both tools to exist, and now that they're free (and/or reasonably priced!) more people can jump in and have fun with what the pros use.