Eclipse is an open source IDE originally developed by IBM. It started as a Java development environment, but has been re-engineered to be a general-purpose development environment. Plugins are available that provide state-of-the-art tools for working with a wide range of systems and projects.
The Proof General Eclipse plugin supports theorem provers using the PGIP protocol.
See:
General Information
NotesAboutEclipse - some useful notes and pointers on PGEclipse for Proof General users