One of my favourite apps for MacOS X is "Omnigraffle" made by The Omni Group (the makers of Omniweb). Omnigraffle is simply a fantastic program. It'll help you create great looking flowcharts, diagrams, family trees, org charts, layouts etc. It's so easy to use and with very little effort you'll be able to create really cool charts and diagrams. I'm by no means any good at doing graphical work on my mac, but I still managed to create this diagram for my dissertation on ecotoxicology using Omnigraffle,- you'll most likely be able to create something waaay cooler..:-)

Omnigraffle has earned itself a 4.8 rating (out of 5) on MacUpdate.com, -a well-deserved rating. You can download a demoversion of Omnigraffle here,- if you decide to buy Omnigraffle AND you're a student, you can get a discount. It's only 41.99$,-and that's a bargain. Check it out!!!