My tool of choice is UMLet, I have used it for a whole year now, and I must say that it works perfectly !
The best points are CLI conversion of UMLet files (.uxf) to other format (.pdf, .jpg, .eps). I use it via a Makefile to integrate diagrams in my LaTeX documents, that awesome :
It's available on the officials archlinux repos, for those interrested (in [community]).
The `problem' is that it's developped in Java, so you have some dependencies that come along..

Also, thanks for this thread venam, I never heard of gaphor, which looks nice !