I really like the Eclipse IDE. For starters, it has the most outstanding installation process I've seen on this side of the Mac OS: Open it. Run it. It works. Why can't all softare Just Work© like that? It also helps that it has an open architecture with lots of plugins available. I'm checking out EclipseSRS, an XML-based, use-case centric software requirements specification tool.

You could almost say that the IBM-funded Eclipse is competing with Sun's NetBeans IDE except, well, they're both free. The only thing they can really compete for is warm fuzzies from the open source community. NetBeans has its own plugin catalogue, but the Eclipse plugins are much more abundant and interesting.