Why on earth would you need a system monitoring tool for your Mac? What's wrong with Apple's own Activity Monitor? Okay, so Apple's tool does a reasonable job of monitoring your system, but atMonitor has all of those bases covered: it'll monitor the standard processor, GPU, VRAM, FPS, disk and network information, all in real time through a series of attractive displays.
More than this, atMonitor lets you go further: you can purge RAM, interact with processes, set up triggering events and log what goes on. And it can be as intrusive or as discreet as you like: get full-blown details in its own window, or banish it to the menu bar or dock for keeping an eye on in the background.
Once installed and set up, the program displays two windows, the main “Top Window”, which displays details of what processes are running, and a floating Activity Viewer window. This mirrors what's displayed in the menu bar, so it makes sense to take a trip into the program preferences to disable whichever one you don't need.
Hard-core system tweakers will find the Top Window of most interest: not only does it allow you to view running processes to see which are taking up most resources, you can select a process to pause or kill it, adjust its priority (click the Renice... button) or send it a signal. You can also create triggers, which activate scripts when predefined values hit a specific target – for example, receiving email notification when CPU activity goes above a certain value.
All monitored values can also be logged, with logs saved in the universal XML format, while advanced users will find a set of useful and domain-specific system commands that display detailed information about core areas of the system – just look under the System Info > Utilities menu.