I've spent the last few months browsing for and testing useful Eclipse plugins.
If one were to divide the plugins into commerical and open-sourced, there's one striking difference between the two groups: As usual, it is the case the commercial plugins have a better finish, and seem more robust, provided they actually start. With the open-sourced ones, it is usually the case that they start, but invariably have many minor and major rough edges that make them tricky to use.
I'd like to mention a handful of plugins that are of special note.
pydev - a Python editor plugin. It provides clean and nice syntax highlighting for python code, as well as an outline. As of version 0.50, there's no completion support (annoyingly, Ctrl-Space shows the list of authors), and some keyboard combinations will irreversibly delete text, bypassing the undo stack.
colorEditor - a reimplementation of the jEdit syntax highlighting engine inside Eclipse, making available all 30+ jEdit modes. I've also tested third party modes, such as the Scala jEdit mode with success.
There are some problems with advanced highlighting cases, that prevented me from implementing a good Stratego-mode, though. The author told me he'd look into it when he got the time.
ecletex - a LaTeX editor for Eclipse. It does syntax highlighting, basic completion, outlining and spellchecking. It's being actively worked on, but is already good enough for the most basic paper-writing.
CDT - a C/C++ environment for Eclipse. I found this to be very unstable, even the 2.0 release. Stay away.
esharp - a C# editor plugin for Eclipse. Pretty slick, but far from JDT. Haven't used it all that much, since I haven't bothered with C#.
myEclipse - a webapps development kit. I only use the HTML and CSS editors, which I find to be pretty okay. The HTML editor is a two-paged editor with a preview in the other page, using the Mozilla embedding. Content assist, formatting (W3C Tidy) and verificiation all work nicely. Commercial, though, but I guess it's easily worth $30.
Very iffy installation, requiring (almost) no end of path trickery.
Visual SlickEdit - the well-known commercial C/C++ refactoring editor, as an Eclipse plugin. Didn't get it to work, no matter what kind of tricks I tried to play.
