wp-cache and addthis

way too many to list, xD

WP Ajax Edit Comments is useful, as are the one's mentioned above. Lightbox is almost essential if you post image galleries.

he asked the more generalized question.

a new thread will detail more

uses one that detects spam and sends a 20,000v shock through the spammer's keyboard.

Beyond "theusual", I use Flickr Photo Gallery, and WPG2 for my Gallery2 installation.

you know... rss aggegator of monkeys at random zoos making looove....

redirection log, ultimate tag warrior, etc