Over at the Wikitosh! Wiki there is a great list of "Must Have" software for the Mac. The list is fairly extensive and provides links to download sites with brief overviews.
I actually use a good amount of this stuff. Some of it is fairly obscure, but honestly useful as well. There are some great tools and utilities you might not know about on there. For more you can refer to Keith's Top 10 Mac Applications.
MustHaveSoftware [Wikitosh!]