OmniGraffle is the only diagram editor I've ever been happy with. It's the one OS X app I'm annoyed to not have an equivalent for on my Linux box at home...
It's amazing that Omni's products aren't copied / used as inspiration more - e.g. it's not that using OmniGraffle as a model for how the UI in a diagram editor ought to act would be all that hard... I wish the Dia guys would spent 10 minutes picking ideas from OmniGraffle, for example..
