Sketchup
I've recently been visiting my friend Paul who is in charge of ICT at a local secondary school. He has begun using a program called Sketchup with his students. Sketchup is a 3D sketching software for the conceptual phases of design. Paul and his students have been using it to develop 3D models of their school buildings, and have created some really interesting 'fly through' views of different buildings etc.
Sketchup is available to download free from Macs or PCs, and there is a useful video tutorial , or a series of downloadable self-paced tutorials to help you get started.