SketchUp
http://www.sketchup.com/ - 3D software from Google.
Features
- Basic version is free to download, but not open source.
- Easy to use
Ubuntu Installation
- 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