If you're a MediaWiki user, the PHP::Impact blog has a list of twenty extensions you don't want to miss out on:
MediaWiki is a great PHP-based Wiki application that is used to power many sites, including Wikipedia itself. One of MediaWiki strengths is how easy it is to extend with its plug-in architecture. The following is a list of 20 extensions that should make your life easier and save you a fair bit of time.
Plugins included in the list are things like:
Check out the full list for more great plugins.