Gate Effects

Gate effects mute the output signal when the input signal is below a threshold level. Some noise gates include a loop, so they can mute the signal after the loop using the signal before the loop. This keeps noisy high gain pedals from buzzing and popping when nothing is played.

DOD FX30 Noise Gate
Gaines Noise Gate - Direct link to pdf. This is a long article with quite a lot of information about both the circuit and general application of noise gates.
ISP Decimator
LPM-23 Noise Gate - Direct link to (japanese!) pdf. See here for more info.
MXR Noise Gate also found here and here .
One without a name
Q & D Compressor - Can be used as a noise gate too.

General Information

Thread about noise gates - With some links to more information.