Processors (CPUs, Processor Adapters, CPU Upgrade Modules...)
Memory (error handling, presence detect)
Storage (Floppy, Hard Drive, CD-ROM, Tape...)
   SCSI (adapters, RAID)
Video
Sound
Keyboard (all hail the Model M!)

Ports and Peripherals
   Keyboard Port
   Mouse Port (YMMV)
      Optical Wireless Mouse
   Parallel Port (Modes, ExpressPrint)
   Serial Port
   Wrap plugs (For diagnostics)
   PC-2-PC Cables (Serial and parallel LapLink cables)
   Joystick Metrics

RTC - Real Time Clock
   PS/2 CMOS Batteries (by Bob Eager and Peter Wendt)
   Reworking the Dallas RTC (thanks to Peter Wendt)
   MC146818 (Clearing the RTC, HW vs SW clock)

Flash ROM memory (late PS/2s only)

IBM PS/2 ASICs List of IBM VLSI Application-Specific ICs