Advanced Google features
I recently posted about some things site owners should keep in mind to make their site more user-friendly and also google-friendly. But as a web user, it is also important to learn to make best use of google, by using some of its advanced features.
Ned Batchelder mentioned the new "definitions" feature.
Other great features are listed on Google's Web Search Features page.
Google's Advanced Search page also lists some tricks like the synonyms search (using '~').
Update: Cory points out another Google keyword "allinurl:".