the UDK or unreal developpement kit is like a Xmas gift form epic.
I was struggling between finish a decent framework in xna or use a game engine when this news came out of nowhere.
Just one week after Unity becomming free, UDK is released and it's huge.
I wasn't really interested in the Unity because of its lack of render to texture function( no shadow, env map or post processing effects) and building tool from scratch is really time consuming. As game developper you prefer spending time on gameplay rather than tools.
I try moding with the cryengine ( since I own the game) and I was pretty impress but the fact you couldn't publish the game and share it to rest of world was a bit depressing.
So this middle ground solution was excatlly what I was wishing without believing it may actually come true.
So how much for that ? It's a bit tricky but it's free until you want to ship it.
but rather than explaining in my terms this is the link
be sure to check the licensing.