architectual drawing board for OSX
OmniGraffle is very popular. A quick Google search just now also came up with yEd which I found interesting (and it is cross platform). 

I don't know your definition for "prettiness" but back in the days when I used Visio I was surprised how much visual effect you could achieve with simple vector graphic tools and a bit of fine tuning. If the built-in tools are not sufficient you can always switch to a pure bred vector graphics program and add a few eye candies.