Sketchup

Sketchup is becoming widely used by designers in the games industry for creating 3D visualisations of level designs.

Originally designed as an Architect's sketching tool, the workflow allows you to very quickly build up 3D space by drawing and extruding various elements.

Best of all the basic version is now free since google bought the product:

http://sketchup.google.com/

Personally I would never use it for building in game geometry, even though the pro version is capable. Maya, XSI and Max are much more powerful tools for building geometry. We use it at Ninja Theory as a tool to help build documentation and to provide a walkthrough of the level for the whole team.