Over on Colloquium there is a list of some great plugins that I think people should be using. I didn’t write the list, but I have to admit that over ninety percent of what is listed are plugins I either use, want to use, or think are great.
There are zillions of WordPress plugins available, with many new ones released each day. Some are well known and almost universally employed, e.g., Akismet, Google Sitemap Generator.
Things listed include a feed copyrighter plugin, and something to truncate super long links in comments. Definitely worth checking out.