Trimble Sketchup. Here is where you get the program:  www.sketchup.com

The best online community is Sketchucation: www.sketchucation.com

Here are some books:

Sketchup for Dummies, Aidan Chopra (Website: www.aidanchopra.com)

The Sketchup Workflow for Architecture Michael Brightman

Rendering in Sketchup, Daniel Tal

These two guys have a common website: (www.brightmantal.com)

Architectural Design with Sketchup, Alexander C. Schreyer (www.sketchupfordesign.com)

Bonnie Roskes, Numerous books on Sketchup, Layout, and Plug-ins: Website www.3DVinci.net