Omnifocus, (Mac App Store - $79.99), is the ultimate Mac nerd getting things done app.

I have yet to take the plunge, but I know that if and when I do I'll be relying on screencasts like these from David Sparks to figure out how to really make the most of Omnifocus.

When combined with OmniFocus for iPhone ($19.99) and/or OmniFocus for iPad ($39.99), you'll be getting so many things done you won't know what to do with yourself.