I can’t find the old thread, so I thought I’d post my list and let other people contribute. More than one Unity user has bought a Mac specifically for Unity, so these lists are useful.
In no particular order (except for TextMate):
TextMate
The best text editor there is.
FinderPop
Nested contextual menus for navigating your files.
Quicksilver
The most un-Mac-like Mac-like program ever made. Extremely fast access to all sorts of functionality and information. The interface is based on recall rather than recognition, which takes some getting used to, but I think it’s a great compromise between the Finder and bash.
lotsawater
My favourite screen saver.
Transmit
The best FTP client.
TheUnarchiver
Faster than the built-in unarchiver, and it means you’ll never have to deal with horrible RAR utilities or Stuffit. The exceptions are password-protected RARs and .sitx files, both of which are fairly rare.
xGestures
If you use a mouse, this program lets you bind gestures to menu items and other actions. Another recall vs. recognition thing, but so fast it hurts.
MenuMeters
For the obsessive-compulsive in all of us. Little status displays for your menu bar. Actually useful for seeing whether your network is working, your memory is filling up, or something is pegging your processor when it shouldn’t be.
Growl
Notifications from a lot of different programs pop up in bubbles of configurable obtrusiveness.
Default Apps
Set default handlers for any file or data type. An unfortunate necessity until Apple gets their file type handling system out of the stone age.
Sleep Display
In conjunction with Quicksilver, this one is a simple, fast way to sleep your display. Especially nice if your computer is in the same room as your bed and you have torrents on the go.
Inquisitor
Find-as-you-type Google search for Safari. Embarrassingly fast compared to Spotlight.
PithHelmet
Removes ads from web pages. Removes ads from web pages! I hardly ever see more than Google text ads when I browse the web.
Adium
Multi-protocol chat client. Unless AIM (and thus iChat) meets all of your IM needs, Adium is a must-have.
Colloquy
Simple, basic, Mac-like IRC client. Great if you want to hang out and ask questions in #otee-unity on Freenode.
Celestia
If you are even remotely interested in astronomy or just getting a feeling for the size of the universe you live in, give this program a spin.