One of the best apps I have ever purchased is Omnifocus by the Omni Group. When I was trying to figure out how to better manage and organize my tasks and projects, I bought every task management program from the cheapest up to the most expensive (Omnifocus), in that order. Once I got to Omnifocus, I started using it and just haven't stopped (Multiple years later).
I am definitely looking forward to the new version when it comes out!
http://www.omnigroup.com/blog/entry/anticipating-omnifocus-2
