A CMoy is a pocket headphone amplifier originally designed by Chu Moy on the headwize.com Headphone Resource Site and public discussion Forums.
The headphone amplifier is designed around single or dual-channel op-amps such as Burr-Brown's OPA2134 or OPA2132PA, however, a wide variety of op-amps have been successfully implemented. The amplifier's design is quite simple. It consists of only a few components, can be assembled on a small section of protoboard, has a lower parts cost than other headphone amplifiers, and can run for many hours on a single 9 volt battery. In spite of this, a well-made CMoy amplifier can drive even demanding headphones, like Sennheiser's HD600 or low impedance Grados quite well. These characteristics have made it very popular among DIY hobbyists, especially those who have not built an amplifier before. The CMoy headphone amplifier can be fit into Altoids tins and can often be found for sale on eBay by searching for "CMoy".
(Photo from Wikipedia)