http://www.waterandstone.com/open-source-cms-resources/articles/50-top-joomla-extensions
I'm re-posting this from another site, who compiled it from a book mostly
Submitted by mariza on Thu, 2009-06-25 11:00
With more than 4,000 Extensions in the Joomla! Extensions Directory, one of the most daunting aspects of selecting a Joomla! Extension is finding the right tool for the job. With thousands of Extensions to choose from, you are sometimes faced with multiple options that appear to achieve your goals. While there really is no subsitute for downloading things and trying them out yourself, in this article I provide a list of fifty Joomla! Extensions as a starting point for addressing common needs.
This article is excerpted from Ric Shreves' upcoming title, the Joomla! Bible, from Wiley & Sons. That book is due for publication in early November and can be pre-ordered directly from the publisher at www.wiley.com. Watch this site across the coming months as we preview more from this new title. This article orignally appeared on the author's site, RicShreves.net.