Mac OS X Tiger/Utilities/Activity Monitor
Activity Monitor is a utility integrated into OS X that allows you to see what is happening within your computer - specifically in regard to your processor, hard disk, memory, and network. Activity Monitor lets you identify and administer the applications and processes running on your computer. It also allows you to observe CPU activity, as well as your use of your memory, disks, and network. The activity can be viewed in the program itself or an icon representation in the dock.
Equivalents on other operating systems
Windows Task Manager (only on Windows operating systems that are based on the NT architecture, such as Windows NT, Windows 2000, Windows XP, Windows Server 2003, Windows Vista)