What would be the "best" drawing tool for OSX that both lets me quickly and freely lets me sketch out ideas for software architectures but also refine and "prettify" these sketches so they can be used in a specification document or book. Sort of a bit like Microsoft Visio but better. With all the predefined shapes and cliparts but also with a little bit more freedom in drawing, layout and design.
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.