NetNewsWire: Without a doubt the application that gets the most use on my Macs is NetNewsWire. At the moment I am rocking 673 feeds and no other newsreader can handle them all in the manner that I like. I've tried NewsFire, Vienna, Safari RSS, and Bloglines. NetNewsWire takes whatever feed I throw at it and asks for me. It is organized well and thanks to the syncing via NewsGator all my feeds are the same across my multiple Macs.
Optimized FireFox: Blogging and browsing the web go hand and hand. If NetNewsWire is that most used app on my Mac then FireFox is a close second. But I don't use any old version of FireFox. No, no. I use the optimized recompile that Neil Lee puts together. Thanks, Neil!
MarsEdit: Another Ranchero app in this list?! Yep. MarsEdit is a simple, straightforward blogging client. You can upload pictures, assign categories and blog using a number of different HTML rendering options (including my favorite, Textile).
Ecto: What's this? Another blogging client? That's right, that's just how much I blog. If MarsEdit is a streamlined blogging tool than Ecto has everything and the kitchen sink (plus there is a Windows version). Ecto has a few nice touches like Amazon.com searching (so you can easily blog about anything they sell), iTunes integration, permalinks are available within the application itself (which I hope is a feature MarsEdit copies at some point).
TextMate: I don't want to continue the 'Text editor holy wars,' but TextMate works well for me. Bundles make TextMate a Swiss Army Knife of text editing. Projects let me keep a number of documents in on file, and the UI doesn't get in my way. I composed this very post in TextMate. What more do I need to say?