I'll try not to repeat any of the previously mentioned apps:
Growl - A notification tool that works with many applications. It has become an indispensable component of my day-to-day computing.
LittleSnapper - A fantastic program for capturing screen grabs. It's more versatile and powerful than the built-in grab. I use it for all my screenshots.
iStat Menus - Good for keeping track of network/bandwidth, CPU, and memory usage, and well as monitoring the temperature of your hardware components.
Acorn - Image editing and drawing program. It's not as powerful as GIMP or Photoshop, but it is easier to use and typically does the job. The thing I love most about it is that it feels like Mac software is supposed to: stylish, easy, and user-friendly, but with plenty of power under-the-hood.
MacJournal - Journaling and blogging software. It lets you post directly from the app in several of the popular blogging formats. I use it to keep track of what I'm writing for whom -- you can switch between entries with a single click, and everything you've written can be put into instantly accessible categories. Really, it just takes the strain out of file management, which lets me focus on writing stuff.
Mactracker - You can use it to record details about all the Macs you own. But for me its value lies in the database of previous and current Apple products, which is impressively detailed and a great reference.
MacVim - A Mac-native version of Vim. If you don't know what Vim is, you probably won't care, but this is great for some quick coding or for projects that don't need an IDE. It supports tabs, Mac-standard keyboard shortcuts, and full customisation.
OmniDazzle - It lets you draw on the screen or highlight certain areas. 'Nuff said.
OmniDiskSweeper - Helps you get rid of all those nasty cobwebs lurking around in the dark corners of your system.
OmniWeb - Yet another product from The Omni Group -- you should get all their software. I use this as my secondary web browser. It would be my primary, but it has fallen behind on a few web standards, so until it gets an update OmniWeb will be my second choice.
Squeeze - It uses HFS-compression to give you a little bit (or a lot) of extra hard-drive space. There is an imperceptible performance hit to files and programs compressed by Squeeze. It claims to have saved me approximately 34 gigabytes of space.
Twitterific - A lightweight desktop-app for twitter that has integrated Growl support.
Times - To be honest, I only use this because I got it free with a MacHeist bundle. It's an RSS reader with a nice interface, and it's good enough for me to not bother checking out the alternatives.
WindowShade X - If you used a Mac in the 90s, you'll know what this is and why I'm recommending it. The main thing it does is let you collapse a window into just the title bar, rather than always minimising to the dock. It also has a few other handy features. I have it disabled right now, though, because I can't spare the US$15 for a licence.
WriteRoom - Distraction free writing. It's basically TextEdit with a proper full-screen mode and a more customisable view.
EDIT: Missed one.
PhoneView - Gives you access to your iPhone/iPod Touch/iPad filesystem, which makes it easy to copy stuff off the device to your desktop or laptop. It's another one that I got in a MacHeist bundle.