Also terrific, but less focused on pure productivity: Adium, Colloquy, Fission.
Great utilities for maintaining your system: AppCleaner, AppFresh, MainMenu, Service Scrubber, Name Mangler, OmniDiskSweeper, Winclone, Tunnelblick, Toast. Growl comes in handy, too.
You might also find Leap useful, depending on how you like to organize your data.
In terms of text editors, TextMate and BBEdit (TextWrangler is the free alternative) have their adherents. So does SubEthaEdit for collaborative work. Some people like MacVim. I personally use Emacs.app, easily built from the emacs source tree.
If you do any serious hacking, MacPorts is terrific.
LaunchBar or Quicksilver: tough call. I haven't seen any Quicksilver updates in about a year, since the original developer open-sourced it. Supposedly a rewrite is in the works. In the meanwhile, I switched to LaunchBar. It's a little more responsive and stable, although not quite as feature-rich or as good-looking.
It's brilliant. If you haven't already, check it out now: http://www.skitch.com
Instead, how about: - Awaken: alarm clocks can be beautiful. - Coversutra: awesome and functional iTunes controller. - Screenflow: Probably the closest (functionality-wise) screencasting client to Camtasia Studio on Windows. - Simple Comic: an bare but perfect (for me) comic/ebook reader that just works.
Life without DTerm... why, it seems absurd!
Also Versions (the best SVN client ever), Transmit (the best FTP client ever) and Coda (a pretty awesome website editor).