Almost anything.

Devices:
LDR's (light dependent resistor), Solar Cells, Photodiodes, APD's (Avalanch Photodiode), PMT's

Special photodiodes:
Quad cells
Lateral Effect Photodiodes
Linear arrays
2D arrays, CCD's
Integrated video camera chips
Pyroelectric devices

Modes of use:
Focal plane imaging/tracking
Occultation
Range by Intensity of reflection
Range by triangulation
Range by phase slip of CW modulated light
Range by pulsed time of flight

I wish I could sum it all up in a little paragraph that would fit here. But I can't.

http://web.mit.edu/professional/summer/courses/engineering/16.90s.html