This is nothing new, but I've recently been using Google SketchUp for making project plans. SketchUp is a pretty easy to use 3D modeling program. It works on both Macs and PCs, and best of all, there is a free version.
Here is a work-in-progress version of the platform bed I recently made. I used SketchUp to plan out the materials for the frame. As you can see, I started working on the headboard but didn't get very far. The program is fantastic for laying out materials. With my design in hand, I knew exactly how many 2x4s and 2x10s I needed to pickup.
I'm currently working on a design for a rolling tool cabinet. I'll be posting that on here when I progress a little further.