Most useful stuff: OmniOutliner, Scrivener, OmniGraffle, OmniFocus, LaunchBar or Quicksilver, Path Finder, and VMware Fusion.<p>Also terrific, but less focused on pure productivity: Adium, Colloquy, Fission.<p>Great utilities for maintaining your system: AppCleaner, AppFresh, MainMenu, Service Scrubber, Name Mangler, OmniDiskSweeper, Winclone, Tunnelblick, Toast. Growl comes in handy, too.<p>You might also find Leap useful, depending on how you like to organize your data.<p>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.<p>If you do any serious hacking, MacPorts is terrific.<p>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.