SketchUp is Google's free 3D drawing program. It's easy to use, but it's even easier when you watch someone with a lot of SketchUp experience use the application and explain what they're doing.
Aidan Chopra is an experienced SketchUp user. In fact, he wrote Google SketchUp for Dummies. (Here's PC World's generally glowing review of Chopra's book.) He also created several videos available on his YouTube page that will get you quickly up to speed on the way SketchUp works. Link
I forgot to mention that John Park wrote a great how-to for Make on designing a workbench with SketchUp. He also has a video on Blip.tv showing the process.
Mark Frauenfelder is the founder of Boing Boing and the editor-in-chief of MAKE and Cool Tools. Twitter: @frauenfelder. His new book is Maker Dad: Lunch Box Guitars, Antigravity Jars, and 22 Other Incredibly Cool Father-Daughter DIY Projects