Eclipse

I highly recommend checking this IDE out.

It is brilliant!!

http://www.eclipse.org/

The Eclipse platform, when combined with the JDT, offers many of the features you'd expect from a commercial-quality IDE: a syntax-highlighting editor, incremental code compilation, a thread-aware source-level debugger, a class navigator, a file/project manager, and interfaces to standard source control systems, such as CVS and ClearCase.

Eclipse also includes a number of unique features such as code refactoring, automatic code updates/installs (via the Update Manager), a task list, support for unit testing with JUnit, and integration with the Jakarta Ant build tool.

Despite the large number of standard features, Eclipse is different from traditional IDEs in a number of fundamental ways. Perhaps the most interesting feature of Eclipse is that it is completely platform- and language-neutral. In addition to the eclectic mix of languages supported by the Eclipse Consortium (Java, C/C++, Cobol), there are also projects underway to add support for languages as diverse as Python, Eiffel, PHP, Ruby, and C# to Eclipse.

Platform-wise, the Eclipse Consortium provides prebuilt binaries for Windows, Linux, Solaris, HP-UX, AIX, QNX, and Mac OS X. Much of the interest in Eclipse centers around the plug-in architecure and rich APIs provided by the Plug-in Development Environment for extending Eclipse. Adding support for a new type of editor, view, or programming language is remarkably easy, given the well-designed APIs and rich building blocks that Eclipse provides.

With hundreds of plug-in development projects in progress, industry giants like IBM, HP, and Rational (just acquired by IBM) providing resources, and design heavy-weights like Erich Gamma helping to guide the process, the future indeed looks bright for Eclipse.

I discovered Eclipse while learning Java earlier this year. I was astonished at its quality... I whole heartedly recommed this IDE to any Java developer (or, as Rendy pointed out, PHP/Python/C++/Ruby etc etc developer ).

Defenitively agree with that. I use eclipse now too, it's great. IBM finally made something userfriendly.

I think Borland is starting to get nervous too...

IBM's websphere is wholly based on the eclipse platform.
I've used both websphere and eclipse by itself. Unfortunatelly for what I was doing (JSP, bean development) it was horribly slow. Websphere was the worst resource hog of all, taking literally seconds to do auto-complete and refresh directories.
I wasn't on a slow machine either, 1.4Ghz with 512 megs of RAM windows 2k.
Anyways I liked the features but hated how slow it ran, websphere automatic XSLT tools are seriously cool.
However, I've since moved to Netbeans, which is also free and very feature rich. Much faster and has tomcat built in. Really good for Bean and JSP development.
Anyways, just my 2 cents.

Cheers,
Benjamin