NetNewsWire - My main source of information from the web comes through this RSS Aggregator. I've tried several different options, both software and web based and keep coming back to NetNewsWire. It does everything I want it to and it does them simply and easily. I use this more than anything else. I use a collection of browsers (which as long as they have tabbed browsing are interchangeable) for other web reading but I'd say at least 80% of what I read online comes through here.
Quicksilver - This launcher application is invaluable. It's made the Dock almost useless and saves me an incredible amount of time every day. A simple keystroke and I'm switching from one app to another without having to use my mouse. This is especially useful since all mice have really been hurting my arm recently.
Instiki - It's a wiki that runs on Mac OS X's built in web-server rather than online. It's not so much for collaborating but it's great for personal notes. This replaced stickies entirely and half of what I use BBedit for. Now I put all my notes and info here. Running the app (either via Quicksilver or the menu-bar option) opens a web browser to this page. It's fantastic.
Adium - Cross platform IM client that covers all the bases. I use it for AIM and Y!M, but also has MSN, ICQ and other options. The buddy-list is super customizable and I keep an almost transparent version on my desktop at all times. It's not in the way when I don't need it, and just glancing at it will tell me who is online. I'm trying to do more and more communicating with IM and less and less with e-mail so something that ties everything together like this is amazing.
X-Chat Aqua - One of my first introductions to the internet was IRC and I was a regular on several channels before I ever had an e-mail address. But that was in college and I thought everyone moved on to other things and until last year hadn't touched IRC in almost 10 years. Joi keep talking about it and I decided to ga back and take a look and it's clearly still a very valuable tool. IRC is the meeting room format of IM. This is helping the move away from e-mail as well.