I always made fun of maczealots, poked at the linux people, and stuck my nose in the air at people who only knew windows. However, I now have to eat all the words I ever said against apple fanboys. If it wasn't for games, I'd never leave OSX. Not only is OSX all shiny and slick, but all the things I thought were just eye candy actually end up increasing my productivity. Coverflow for folder browsing, Expose as an app switcher, tagged files searchable through Spotlight, Multitouch for touchpad interaction, not to mention a fantastic screen and great keyboard. Here's a quick list of the apps that I have come to use on a near daily basis.
MS Office 2008 - yes yes yes, I know. As a converted macdude, I'm supposed to hate everything made by M$ (or whatever the cool kids call em nowadays). Word and Excel still can't be beat. And I still hold Visual Studio for .NET as a shining example of what a modern IDE should be.
iWork - I haven't found a need for Keynote yet, but I do use Pages on occasion. Pages isn't as full featured as Word is, but it is much much easier to write a pretty document in Pages. Case in point: I made my newest resume using Pages, and exported it into a *.doc.
Eclipse - Not as good as Visual Studio, but getting closer.
Terminal - OSX's terminal does all the good things that terminal apps other *nix os's do, tabbed sessions, etc. Not to mention all my familiar *nix commands which I can run. Indeed, I manage all the application servers at the office from Terminal.
Safari - not as good as firefox, but I need my Multitouch, which only works on Cocoa apps. No firefox extensions makes me a sad panda.
Omnigraffle - WOW. Just as full featured as Visio, and extremely well put together. I use Omnigraffle for mind mapping, especially when I have to learn new things.
Omnifocus - double WOW. I've read all about the GTD (Getting Things Done) fad, I thought it was just for people who don't know how to use post-it notes or a text editor. While I don't have to eat my words again.... I may have to nibble them.... just a little.
iTunes - Meh. I liked using Media Center in Vista better. Media Center's "album view" is just as pretty as Cover Flow, and lets me see more albums at once.
VLC - Oh yeah!
TrueCrypt - This app is a must. Seriously. The entire world should be using this.