Where Do I Find Plugins?
A quick Google search will yield a surprising amount of results for Joomla extensions, but you should be wary. Not all extensions are fantastic, and some aren't compatible with certain versions of Joomla. It's important to learn which version of Joomla you're working with. (I recommend 1.5!) After you figure out what version you're working with, most of the hard work is done. Now, it's just time to crawl some websites to find which plugins you might want. Here's a list of a few websites that I personally enjoy.
Joomla Extensions Directory
This is the official Joomla extension website, and it's absolutely packed full of great official, as well as third-party extensions. You'll easily be able to find a plugin to handle any function you need! And one of the best things about this website is that not only will you find high-quality third party extensions, but there are hundreds that are free of charge. Free plugins for Joomla? Awesome!
YOOtheme is a long time favorite of mine. While they're not free by any means, they are relatively well priced, and the extensions (YOOtools, as they're called) are the highest quality you'll find. There are dozens of plugins to handle image effects, image galleries, Twitter widgets, and even logging in.
RocketTheme's Rok Collection
If there's one thing that I've been kicking myself about not finding sooner, it has to be RocketTheme's Rok collection of plugins and extensions. RocketTheme is definitely what you want to use if you're into alternative site design and you want some reliable, cool plugins to jazz up your site.