Joomla extensions come in three flavors - Components, Modules and Mambots. Some extensions require one from each category to run
AkoComment - if you're using a blog - this is a great comment component
Artio JoomSEF - Creates human friendly URLs
Community Builder - Overrides user system to create communities including profiles etc.
CB components are called plugins - they are separate from normal extensions in Joomla
Some good 3rd Party Plugins - Plug in page
CB Captcha Plugin
Joogle (geocodes members' addresses and adds them to the google map)
Jim - IM within profiles
subscribemailman - email list subscription service
several gallery options
Disable Core Regisration - goes with CB
there are 88 plugins listed on the Joomla site...
EventList - An event calendar - produces easy list(you'll need an add-on) and allows for regristration at events. (you'll want to edit the logo)
RD RSS - Allows you to create RSS feeds for any section/category - unfortunately - the site is all in German - but the component is easy to figure out.
Simple Image Gallery - provides thumbnails and lightbox for any directory of images.
Tabs & Slides in Content Items - Tabs in a content item.
AllVideos Plugin - play just about any video in a content item.
All-in-One VideodePoche Buttons 2.5 Buttons to easily insert AllVideosPlugin in a content item
Ajax Header Rotator - displays a series of images in the header
JCE Editor - "best" wysiwyg editor for Joomla.
Jumi - allows for PHP code to be run in a module or content item
Related Items XTD - allows the addition of related content in a content item
Gallery 2 Bridge - Joomla interface for the Gallery App
Google AdSense
Google Analytics
MosIf - MosIf is a conditional plugin (mambot) that allows you to dynamically control what the user sees. When you publish some content in Joomla, all you can control is what kind of users are allowed to see the content. You can choose between public (ie. all users), registered (all logged-in users) or special (like administrators). If you need more control that this, MosIf can give you much finer control of the content.
Content Item Module - his modules allows for publishing content items or articles in areas other than mainBody().