Activity Monitor

Where to find it: in the Utilities subfolder, inside the main Applications folder.

Activity Monitor is a useful utility for Mac OS X that is included in OS X versions 10.3 and 10.4. It allows you to view every process, or task, that your computer is currently running. Along with the name of the process (for example, "Safari" or "iTunes"), you can also view helpful information like how much of your system's resources it is taking up. This can include how much memory it's taking up and the percentage of the computer's CPU it's using. (Process is a general term for "task," and can include both applications, like Safari or iTunes, as well as other things your system is doing.)

One of the most useful things Activity Monitor can do is help you deal with unruly programs. For instance, if the program you're using suddenly becomes unresponsive, it may have crashed - and you can use Activity Monitor to check what's going on. Open Activity Monitor, and look through the list of items until you find the name of your program. If the program has actually crashed, its name will probably be highlighted in red with the words (Not Responding) to the right. Also, the process will also probably be hogging system resources; look to the right of the Process Name column and look at the number under the % CPU column. This is the percent of the CPU the process is using - if it's in the high 90s, it has probably crashed.