Photography and Recording
Many shows require a photographer for their publicity material and also to photograph the show itself for archival purposes. Video recordings, which can be transferred to DVD to sell as memorabilia or at a profit, are also quite popular. Audio-only recording services also exist, for soundtrack CDs and "radio play" formats.
Needless to say that you must have the consent of your show's copyright holders to photograph or video-record it, otherwise you may get into legal hot water. You should also speak to the venue well in advance - if you turn up on the night and expect to record the show then you are liable to be thrown out by the Duty Manager!
Photography
The Cambridge Theatre Photography Facebook group is an excellent starting point.
Claude Schneider is a Cambridge grad and a professional photographer specialising in portraits/headshots, show/concert photography, publicity design, and much more.
David Hone is a Cambridge-based photographer, happy to do dress rehearsal and performance photography for free. Can provide prints and a web-based gallery.
Loren Miles is a University staff member who is happy to do dress rehearsal and performance photography for free.
DVD Production
Redbox Productions work includes Return to the Forbidden Planet, Faust the Panto, A Midsummer Night's Dream, Macbeth, Sprung! 2007 and many more.