We joke quite a bit here in the office about how many WordPress plugins there really are. Currently in the WordPress Repository, there are well over 6,000 plugins for download. Although I would not recommend all of them, there a tons of excellent plugins that can make your site do pretty much anything you want. Of course, I have a few favorites.
Preface: I realize that two of the plugins on the list are written by a coworker of mine, Chris Jean. Funny thing is, I have been using his plugins since before I knew him. They are amazing!