Mac OS X: atMonitor is a free system monitoring tool that gives you highly detailed, real-time information about your Mac, running processes, and just about any system information you could want to know about.
atMonitor is like your Activity Monitor — the free utility that comes with your Mac — on steroids. Not only do you get additional information in the main application window, but you also get a floating HUD with an overview of your system activity and menubar stats as well. One of the coolest features of atMonitor, however, is that you can assign a script to run if any process reaches a certain level.
If you're looking for a more powerful system monitoring tool for your Mac, atMonitor should fit the bill. While you can download and use it for free, atMonitor does accept donations should you want to support its further development.