Eclipse is an open source IDE and software framework. Best known as an IDE for Java development, it can also be used for Ruby, Python, C++, and other languages. (The Wikipedia entry provides a good overview.)
Although Eclipse can run on older Macs, I highly recommend the new Intel-based Macs as a platform for running Eclipse. I also recommend that you have at least 2GB of RAM.
Eclipse 3.3.x is the latest release as of this writing.
As of this writing, I am using Eclipse 3.3.2 on Mac OS X 10.5.2. For each version of Eclipse, there are many configurations available. For server-side Java development, I recommend the Europa build of the Eclipse IDE for Java EE Developers.
Eclipse has a large and growing collection of plug-ins. This section lists some plugins that I have used and can generally recommend.
If you are going to use Eclipse for Java development, the Callisto project has collected all of the essential plugins in a way they can be installed together. Eclipse 3.2 has Callisto already configured in the "System Updates" (check this) section of Eclipse. I went ahead and installed all of Callisto, even though there are some parts I probably won't need. It was just easier that way...
Table 1. Plugins