Activity Monitor

If a program is stuck, usually you just press cmd+alt(option)+esc. That bring ups the force quit window. Sometimes, however you need to see more in detail what your computer is up to. Open activity monitor and it'll show you everything you might need to know about the processes running. Down at the bottom you'll see different stats. Although I don't understand them entirely, I know you can change their color. Woo!

Also at the bottom you can switch between different kinds of information, like system memory, disk activity, CPU, etc.

Among the information provided to you is the kind of process (in this case either intel or PPC) the amount of memory, and the percentage of cpu alloted to it, etc.

This application saved my hide once when I though Lightwave wasn't responding, but it was actually just working really hard...

If you need to have a quick glance every once in a while though, you can do this nifty trick.