The following tools are often used for diagnostics on Mac OS X:
Activity Monitor
Apple Hardware Test (included on disc with most current Macintosh computers)
Apple Service Diagnostic (available to Apple Authorized Service Providers)
DigitalColor Meter
Disk Utility
Display Utilities (available to Apple Authorized Service Providers)
MacTest Pro (available to Apple Authorized Service Providers)
Network Diagnostics
Network Utility
System Profiler
There are several third-party utilities that are useful for troubleshooting, including Tech Tool Deluxe (available with the purchase of an AppleCare Protection Plan)