Either it's a recent addition, or I have not noticed it before, but Apple now has now a Macintosh Basic Troubleshooting web site. By selecting your symptom through a series of questions, you (hopefully) eventually arrive at Apple's recommended solution. As its name implies, the information is on the "basic" side, but many users will still find it useful.
Mac running slow?
Boost your computer with these five useful tips that will clean up the clutter.