I thought it'd be a good idea to keep an on-going list of helpful wordpress plug-ins.
THEMES/CODING/ETC
runPHP - plug-in that allows you to run php directly from your posts.
stats - statistics for your wordpress blog by Wordpress.
Theme Test Drive - test new themes without changing your old theme!
FORMS/POLLS/ETC
cforms II - fully customizable forms to put in your posts, pages, sidebar, etc. It's really good :)
WP-Polls - great tool that lets you insert polls straight into your wordpress site without an annoying 3rd party interloper. :D
STATS
WP-useronline - shows you how many users are currently online, who they're referred by, what page they're looking at, etc.
WP-stats - statistics such as: total posts, total comments, most comments in what posts, etc.
SPAM CONTROL
WP-reCAPTCHA- prevents spam in posts by putting up a "captcha."
BAN URL/IP/ETC
WP-Ban - ban people by ip, host name, referral url, etc.
SIDEBAR/OTHER FUN STUFF
Flikr - This plugin is awesome and getting very popular. You can customize how the pictures appear on your page by styling it with CSS. You can see it on my page.
Twitter - I love this plugin. For no apparent reason than because it's really fun.