Best part of using Eclipse is the large plugin developer community. Since it could be a little bewildering, here are some of the plugins I am using with Eclipse 3.0. They are all free and most of them are open source.
Color Editor - Free, Open Source
While Eclipse ships with syntax coloring support for many file types, it does miss some major ones like HTML, JS, JSP, CSS, and even XML. The last one is a surprise because XML syntax coloring plugin was one of the example plugins offered by the Eclipse team. Anyway, Color Editor adds syntax coloring support for 74 file types by porting syntax coloring files for JEdit, a popular pure-Java editor, to work under Eclipse. Very nice.
Sysdeo Eclipse Tomcat Launcher – Free, Open Source
If you do anything with Tomcat, you'll have to get this one. There are several other plugins that lets you do the same, but this one sucks the least among the free ones and the author seems to be still mildy interested in keeping it up to date. JSP debugging is a bit of a hassle, but not quite enough for me to wade into the source code to fix it yet. I am hoping another talented sucker will have less patience than I. Heh.
QuantumDB - Free, Open Source
With this plugin, you can bookmark databases, browse them using a tree GUI, and execute queries on them. Results are displayed in a grid. Not exactly impressive, but very functional and very handy.
Azzurri Clay – Free Core Version, Not Open Source
Clay allows you to edit database schemas visually similar to the way MS Access does it. It's not a tool I use everyday but it's very useful when you do use it. You can create a diagram out of an existing database, make changes to the schema model, and then generate SQL dialect-specific SQL. This is pretty handy for porting your database to another database implementation since Clay will generate the target database specific DDL.
GEF - Free, Open Source
Graphical Editor Framework is a framework for, surprise, building graphical editors like UML diagram editors, etc. This plugin is used by many other plugins (i.e. Azzurri Clay) so you'll have to get it eventually. Only issue I have with GEF is that it doesn't support Java2D yet. Since Eclipse 3.0 adds support for AWT, JFC/Swing, and Java2D, I am hoping this shortcoming will disappear soon.
BTW, I usually go to the Eclipse-Plugins Info site to find plugins. Many of the plugins listed there will have some problem with Eclipse 3.0, but now that Eclipse 3.0 is out, updates are coming at a fast pace already.