Eclipse runs on Windows, Linux and Mac.

Eclipse is an open source project and free to use.

Thanks to the large variety of plugins and various configuration options, Eclipse is very customizable.

Eclipse has a large and active community, which has resulted in a wide variety of plugins.

Shows threads, concurrency locks, and conditional breakpoints.

The concept of perspectives is outstanding. It puts right tools at your fingertips, keeping the tools you currently don't need out from the workbench. For example, in VCS perspective it's all about versions and branches. In debug perspective it's all about state.
In java ee project it can show http endpoints in a very accessible manner.

Eclipse is really fast if you not install all available modules and have enough memory (at least 2GB).

Because Eclipse is based on SWT, it uses the native font rendering and thus looks better than other IDEs on some Linux systems, where the Java font rendering is not optimal.

Eclipse uses a custom compiler (which can also be used outside of Eclipse), which is often faster than the normal Java Compiler, especially for incremental compilation.