Level design tools?

I prefer pen and paper

because, you know.. it's free.

I'm sure these tools are great, but pen and paper is very quick and easy. and generally I find once its into the 3D world the Top down isn't required. The dynamics of the game play change so much once the Z axis is brought into the game.

That said, in a large team where producers like to see these changes take place this may be amazing.