RCX, NXT, WeDO, EV3, Technic etc. How many different motors have been released over the years??
For a comprehensive enumeration and measurements on all motors, refer to Philo's excellent page on this topic: http://www.philohome.com/motors/motorcomp.htm
There are some LEGO motors that are missing from this list since they predate the 9V era, like the 4.5V motor, or the 4.5V train motor, but these are probably only of interest if you already own them, in which case you don't need to be told about them :)
There are other elements that might be considered motors, like the PF servo motor (Thanks to Phil B.), but one might argue that the servo motor is not a motor in the strict sense, since it has a fixed range of movement instead of being able to turn continuously.
Also there are pieces that incorporate motors with other functional elements, like the main part of the 4095-1 Record and Play set, the Spybotics module, the Micro Scout in the Droid Developer Kit or the Cybermaster main unit.
The WeDo theme uses a Power Functions M motor (Source).