Denyer told you already the most important ones.
The following might be interesting too:

Css Editor plugin:
http://plugins.jedit.org/plugins/?CssEditor

PHP Parser plugin:
http://plugins.jedit.org/plugins/?PHPParser

Here is a simple macro to start an external validator:
http://community.jedit.org/?q=node/view/219

To read in the images dimensions in an element, I wrote the ImgTag plugin. It's not officially released but you may want to try out:
http://www.moviecorner.de/imgtag/ImgTag.jar

The plugin called XML is probably the most useful -- intelligent auto-completion for tags as you're typing. It also offers a tree of nested elements (via Sidekick) and apparently even does DTD validation.

Two other possibly useful text-oriented plugins are WhiteSpace and TextTools, and of course there's one to integrate HTML Tidy...