The folks that have created IntelliJ have really done some nice things for those of us that use open source tools. Of course one tool we all use is CVS for source code control, and the IntelliJ/CVS integration is pretty sweet.
Continuous integration is a key to a quality build process for any multi-developer software development project. I can't say it much better than the way Martin Fowler describes it, so I'll just include a portion of his summary here:
Continuous Integration is a software development practice where members of a team integrate their work frequently, usually each person integrates at least daily - leading to multiple integrations per day.