Activity Monitor is a Mac OS X system utility that tracks your computer's activity, such as the currently running processes, CPU load, available memory, and so on. It provides useful graphs for most of this information, including a CPU load meter that you can place in your menu bar or elsewhere on-screen. Activity Monitor can also force-quit troublesome processes, even if they don't appear in the Dock.
BigTop graphs some of your computer's vital statistics, such as CPU load over time. The information is slightly different than that displayed by Activity Monitor. BigTop is part of the CHUD tools installed by Mac OS X's Developer packages.
MallocDebug analyzes a program's memory use as it runs. You can use MallocDebug to find places where your program is reserving memory more often than you expect. You can also use MallocDebug to find memory leaks, places where memory is allocated but never freed.
MONster is a low-level hardware sampling tool that can provide very specific information about CPU, operating system, and subsystem performance. Most application programmers want to analyze their software at a higher level, using tools such as OpenGL Profiler, Sampler, and ...