Dia

One of the best OpenSource diagram editor i know, and I tryed a lot of this tools. Its easy and intuitive to handle and the output looks just great. Even create own custom shapes is as easy as drawing diagrams.

This is a good tool - especially in comparison with what else is out there. I do think that diagrams could have been a bit smoother and nicer looking, but who am I to complain - it gets the job done without any serious hustle.