The internet has a vast treasure trove of resources available to SketchUp user. These are the ones that I reference often:
3-D Warehouse: http://sketchup.google.com/3dwarehouse/ – Googles one stop source for user submitted components. Extremely useful for landscaping components, furniture, vehicles, people, etc…
Smustard – http://www.smustard.com/ – Smustard has a vast collection of SketchUp Plugins, many of them free but also offers paid plugins. A very trusted site that offers many addons for SketchUp
Ruby Library Depot – http://rhin.crai.archi.fr/rld/ – Ruby Library Depot offers a large collection of Ruby Scripts for SketchUp. They have a nice system of Plugins categories that helps the use to find the script that are most relevant.
SketchUcation – http://forums.sketchucation.com/ – Look through the forums and find many useful SketchUp plugins.
Google SketchUp Ruby Samples – http://sketchup.google.com/intl/en/download/rubyscripts.html – Directly from the SketchUp site, Google offers a brief synopsis on the Ruby Scripts and provides some ruby script samples, which happen to be some of the more useful ones.
SketchUcation – http://forums.sketchucation.com/ – SketchUcation forums has many answers to your SketchUp questions answered by professional users from around the world.
Google SketchUp Forums – http://www.google.com/support/forum/p/sketchup – Google’s official SketchUp forums.