I have been using OS X excessively since the purchase of my Macbook, and the thing that drove me to this is the software. Sure there is an unlimited supply of software for Windows, but lets be honest most of its crapware. Sure there are some gems like Windows Live Suite, Alcohol 120%, and the all to essential Office 2003. But for me being a poor college student I can’t afford to buy a lot of software so the open source community is where I get my software. Since I have been on Mac I have found so many great problems like,

Quicksilver
Acquisition
VirtualBox
Adium
Alarm Clock

I cant imagine living without this software now, its really that great. Sure there are Windows equivalents of this software but I now prefer their OS X counterpart. Now before you call me a Windows hater, notice that I have VirtualBox on the list, this allows me to run Windows for the software that I have to use for school and to write this blog, I use Windows Live Writer, its awesome.
So what open source software do you use? It can be on OS X, Linux, or Windows let us know what you think is good.