Note that with the acquision of SketchUp by Trimble, many of these below links may soon be deprecated and changed. We will strive to keep everything on this page updated as much as possible.
SketchUp Home - the starting page for "all SketchUp"
Download SketchUp - the official download page for SketchUp - both Pro and Free versions. Note that there are some places all over the internet that provide download links too but always be careful where to download. Also, here you can always access the latest builds with the most up-to-date bug fixes and new features. This is another place where you can access all current, publicly and officially available versions (e.g. language variations) of SketchUp.
Older SketchUp (Pro) versions - if you are a Pro user, you can download older (and still supported) versions here.
Release notes - for all currently supported versions of SketchUp, LayOut and Style Builder.
SketchUp Help Center - general resources for all official, online help articles, links to the official SketchUp Help Forums (Google Groups) and others. The Help Articles were last released as downloadable PDF versions for SketchUp 7 and LayOut 2. Much of what's written there is still applicable to the current versions but you will need to make sure to compare to your current version's working.
Contact SketchUp Pro Sales - if you are a Pro customer, you can contact SketchUp sales here. If you have licensing problems, you can try the License Trouble Shooter or retrieve your licese inormation here.
SketchUp Technical Support - if you are a Pro user, you can try to send your technical problems to the SketchUp Team.