What is Eclipse?
Eclipse is an awesome IDE for creating apps, in various languages. It has a GTK+ UI that is much more accesible to regular programmers from a Windows environment than the tools traditionally used by GNOME hackers.
It is currently best for developing apps in Java, but support for C, C++, C# and Python is rapidly catching up.
It was originally a (very) large body of code that IBM open-sourced. There is now a large community of companies and developers using and extending Eclipse.
It has its own Open Source license (the Eclipse Public Library, or EPL).
It has some hackers who hang out on #eclipse-dev on irc.freenode.net.
The main Eclipse site is here - it's huge!
Using Eclipse to hack on GNOME
You can use Eclipse to hack on GNOME now - grab Eclipse and the C Development Toolkit (CDT).
Apparently CDT now includes Autotools support, which is the build system most GNome projects use. On the Fedora, an Autotools project support has always been part of eclipse-cdt package.http://www.eclipse.org/linuxtools/projectPages/autotools/
You can add Package config support to Eclipse [http://code.google.com/p/pkg-config-support-for-eclipse-cdt/ here], to allow you to easily add dependancies on Gnome and it's libraries.http://ubuntuforums.org/showthread.php?t=807706
Some hackers at Red Hat put together these useful patches for working with GNOME projects:
This plugin adds PackageKit integration to Eclipse proposing you to install missing parts for you based on various information (e.g. missing pkg-config requires from Autotools projects. https://fedorahosted.org/eclipse-packagekit/
This plugin adds support for ChangeLog editing; you can select files in a project and have a ChangeLog stub generated. It can parse C, Python and Java and handle the diffs against CVS intelligently. Information on grabbing from CVS is here
Gnome documentation integration
Perf integration is available from the Eclipse Linux Tools project. User guide.
Some more notes on extending Eclipse to integrate better with GNOME can be found on /Ideas.