Since I highlighted the Windows version of this Lifehacker list of essential free downloads, I think it only fair that I also include the companion list for Macs, even if I am not personally a Mac user. I will cop to being a Mac wannabe, though, and that counts for something.
There is definitely overlap between the Windows and Mac list, including Firefox and iTunes. There are also plenty of other offerings addressing productivity, communication, utilities, multimedia, and file backup/syncing.
Enjoy the free!