What WordPress plugins are "must have" for a site?

Askimet is a must have for spam control

WordPress.com Stats is great for stats

WP Super Cache, or any other caching plug in, is good for performance

TweetMeme Retweet Button is good for getting users to tweet about a blog post

This is a list of the most popular plug-ins. That's probably because they're very useful in some way.

JS and CSS Script Optimizer - combines your scripts and stylesheets into one document each for minimizing total HTTP requests. The web page generally doesn't render until all of the stylesheets have been loaded, so if you have several attached, this can really improve load time.