
I tend to not use a lot of third party apps on my Mac, simply because the Mac OS X default apps are excellent and fulfill 80% of my needs. The best apps are the ones that just fit in. However, there are a few really nice apps out there for Mac OS X that I use a lot. These ones are all "dock worthy."
And yes, I've worked on a few of these; but that's why developers write apps, right?

OmniGraffleA true digital whiteboard. It puts planning into two dimensions. I use this a lot for school too, when I need to create a flowchart or diagram.

WalletWallet is great for storing all of your personal tidbits. I use it for storing serial numbers, passwords, domain names, email addresses, and much more. And with .Mac import/export, I always have an up-to-date version of my database on any Mac.

Delicious LibraryI love keeping track of my media and seeing it all at once. Sadly, this app is becoming dated and almost requires manual input of information and artwork these days.

TransmitTried and true FTP client; I've been using it since before Mac OS X. If you're a Transmit user, I recommend
this toolbar icon replacement by Adam Betts. It makes using Transmit that much better.

SubEthaEditMy code editor of choice. SubEthaEdit "just works", has great syntax coloring, and of course, collaborative editing.

TypesetVery niche, but great for designers. Before Typeset, I used to flip through fonts manually in Photoshop. Typeset makes font selection a ton faster, and even lets me mark any font as a favorite with ease. It should be in every designer's toolkit.

InquisitorOne of those apps you didn't know you needed until you started using it. Searching without Inquisitor installed simply feels wrong. Made by the very talented Dave Watanabe.
Am I missing anything? What 3rd party apps can you not live without?