There seems to be some useful software here: Open Source Mac – Free Mac software, all open-source, all OS X.
For the unfamiliar, open-source software is software where the authors provide the underlying computer code to everybody. That lets you examine how it really works, compile it to run on a range of machines, and make custom modifications.
Much open source software is free.
P.S. While I am appreciative of free and open-source software, there are a few pieces of commercial software that I really wish I could buy for a bit less money: the latest version of Photoshop, a copy of Office for my laptop, a commercial version of PGP, etc. EndNote would also be useful, as I ramp up research for my doctoral thesis.