The only reason I’ve ever considered buying a Mac is OmniGraffle. This is a really cool program and I wish the company finally released a Linux version. Not sure how close to all the Mac APIs it actually is but that I guess it’s something that can always be worked around as long as there’s demand, and clearly there is.
So a couple of years ago I started evaluating all sort of diagramming applications available for Linux – and I do mean spending a couple of weeks on using all of them – and about a year ago I settled on using yEd as a diagramming tool of my choice. This the only one that looks decent, is fast enough, and – the most important bit – the diagrams look modern, the authors understand that merely displaying crude boxes and arrows isn’t always enough so they keep attention to a lot of details that make it all look very sharp and attractive.
Here is a couple of diagrams I’ve recently made though by no means is it the end word, it’s just a sample of what I’ve created myself, not the ultimate showcase of the program’s features.