Excluding multiplatform apps (
Photoshop — can't live without this on ANY computer) and apps that come with OS X (
Safari, Mail, Preview, TextEdit, etc... I use a lot of them), my list is probably something like:
-
Adium
-
Coversutra
-
Transmission
-
iPhoto — I'll count this as it comes with new Macs but not OS X itself, and is essential, especially if I want to give my iPhone new wallpapers. >.<
I could probably add
Transmit to the list, but I don't FTP a lot at home and don't have Transmit at work.

Or maybe it's because CyberDuck is so slow and crashy.
I also might have added either
Quicksilver or
Launchbar, but since Leopard came out the new Spotlight has been "good enough" to warrant having one less application running in the background.
Also,
Inquisitor is fantastic, though not really an application per se. Same goes for
Safari AdBlock (the Leopard-only one) which is
torturous to live without.
And without
Perian I'd be forced to watch things in VLC, which used to be essential, but is falling behind the times recently.
It's hard to leave
Toast off the list too, cause when I need it, nothing else will suffice.
Yeah, it's hard to keep the list to 4 apps.
