Monitoring your system's processor activity can be very important, and OS X has a great monitor utility called Activity Monitor built in that you can use to kill/tweak processes and monitor network, CPU, RAM, and hard drive use. Once the monitor is running, you can right click on its icon in the Dock, select Dock Icon » Show CPU History, and have your CPU activity show right in the Dock. However, the default background of the dock activity tracker is black, making it plain you're looking at a box; this makes it not quite fit the smooth look of the rest of the Dock. You can change the colors, though, and even make it have what seems to be a transparent look. To do this, fire up DigitalColor Meter and Activity Monitor; both are in your Applications » Utilities folder.
In DigitalColor Meter, change the sampling to
RGB as actual value 8-bit and put your cursor right next to the Activity Monitor dock icon. Press Command-L to lock that location in DigitalColor Meter. In Activity Monitor, click the CPU tab, and then click the black box next to "% Idle." Select the RGB sliders tab (the second tab) of the Color window, and enter the RGB values displayed in DigitalColor Meter.
[
robg adds: This isn't true transparency, of course, just the appearance of transparency. I use
MenuMeters to track CPU usage, as the meter is always visible in the menu bar and I keep my dock hidden.]