Brilliant Instruments

PLLs and frequency modulation measure jitter, time interval error and settling time
Ultrasonic and radar pulse timing
Optical and magnetic disk drive measure jitter and bit timing directly
PCI Express and other high speed serial interfaces measure jitter and skew
Oscillators and crystals measure start-up time and Time Interval Error
Pulse width modulated signals measure variations over time
Nuclear physics
Time stamping of events in real time