OmniGraffle
OmniGraffle kicks. I always thought that Visio was a decent tool for creating diagrams, flowcharts, etc., but after using OmniGraffle a few times, there’s no comparison. Everything about it is absolutely intuitive. If you don’t know how to do something, you just try what makes sense and OmniGraffle will always be one step ahead, doing something that’s even more useful and productive than you expected.