Here is a list of plugins / extensions that I use for deigning my models in SketchUp, best of all most of these be found for free on the internet and most are provide as open source. Many of the authors have graciously provided these plugins for free, although most accept donations for their incredible work. I have attempted to link to these extensions, however most of them are continuously updated through the webpage Sketchucation.com or through the built in Sketchup Extension Manager, so if you happen across a dead link please let me know in the comments below.
(see Sketchup RBZ file support here for what to do with rbz files)
Foam Casualty Airfoil Data Loader (This is now obsolete, an updated and improved airfoil data loader is included with NCGen)
Mirror Selection by TIG
Find Stray Lines by Smastard (used in assistance of 3d tool path creation)
The Unfold Tool by Jim Foltz
And of course we can't forget the Foam Casualty NCGen extension.