Compressor Effects

A compressor changes the dynamics of your playing so that sounds louder than a given limit becomes quieter. You could say it cuts the peaks (volume that is).

Compressor or limiter?

For it to be a real compressor it should also boost the signal prior to cutting the peaks, without the boost it's not a compressor but a limiter.

Discussion about compressors
Supasustain - Schematics and discussion about how it works.
Flatline Compressor
Orange Squeezer
Q & D Compressor - Can be used as a noise gate too.
Modified Q & D
Boss CS-3 - Schematic and explanation.

Sites with multiple effects

Tonepad - DOD 280 Compressor, MXR Dynacomp, Orange Squeezer, Ross Compressor.