WordPress plugin that appends a wikipedia (or other mediawiki site) entry to the bottom of a wp post / page. Uses ?action=render on the end of the url to get just the HTML for the article, not the site. Useful as a resource.
Ryan (core WP dev) Boren's post on taxonomy (category & tag) intersections and unions. WordPress has in-built support for complex include / exclude querying.
WordPress forms plugin, similar to cforms but potentially simpler. Can't find it on WP.org.
Tips on how to both detect WordPress hacks and more importantly, prevent future hacks.
A list of security related plugins for WordPress. Some good stuff.
A directory plugin for WordPress. It allows you to whip up a directory, based on the DMOZ design, for any topic without writing any code. Could be nice.
The Feed Statistics plugin does exactly what it says on the tin, gives you stats on your feeds including subscribers, link click throughs, etc. It seems to do what Feedburner offers.
Pop in a WordPress function name and it'll tell you when it was introduced and when it was dropped. It tracks over 1'900 functions through 30 versions of WordPress. Nice. Thanks ozh.
The Custom Write Panel WordPress plugin allows you to create new "Write" panels in the admin which include custom fields. You can also hide the standard fields from view.