I've been a Mac user for a little over a year now, coming from about 10 years with Windows, so I thought I'd take the opportunity to share the applications available for Mac OS X that I find absolutely essential.
Some of these are software I loved from day one, while others are fairly new finds that improve my Mac experience. The idea here is to only cover applications that aren't bundled with Mac OS X (amongst those, Preview is fantastic, and iMovie and iDVD are a must for importing and creating nice DVDs).
If I could only choose one software to install, it would be Quicksilver. It's an amazingly powerful software, which to some are just an easy way to start applications, but for the power user, there's no end to the possibilities. Install it. Now!
Are you a fan of TotalCommander or NortonCommander on Windows? After many tries, I think I've found the equivalent for Mac OS X: muCommander. It is almost as good as the above mentioned, and a personal recommendation is to open the action_keymap.xml file (located under [your user name]/Library/Preferences/muCommander) and change the keyboard shortcuts to your liking (a hint: "meta" is the name for the Command key).
It also supports FTP, so no need for any separate program for that.
A plug-in to QuickTime Player for playback of WMV-files (a must for web sites who have chosen a crappy streaming video format). It is freeware if you follow the link above in the heading to the Microsoft web site.
Without a doubt, the most competent program for programming on the Mac. Extremely powerful and versatile adaptions available for an abundance of programming languages, the ability to customize it to your needs and bundled with superior functionality. A must, if you're a developer