It is OS X only, not open source, and I haven't used it myself, but I've heard a lot of recommendations for OmniGraffle. http://www.omnigroup.com/applications

It is OS X only, not open source, and I haven't used it myself, but I've heard a lot of recommendations for OmniGraffle. http://www.omnigroup.com/applications/omnigraffle/ http://www.omnigroup.com/applications/omnigraffle/ I was in the market for just such a beast a couple of years back: open source, high quality, cross platform. I'm not sure it exists yet.