SketchUp

http://www.sketchup.com/ - 3D software from Google.

Features

• Basic version is free to download, but not open source.
• Easy to use

Ubuntu

• Install Wine: https://help.ubuntu.com/community/Wine#Installing_Wine
• Download SketchUp for Windows and open it: http://www.sketchup.com/intl/en/download/gsu.html

Tutorials

• Get started with SketchUp - Part 1