Orange Squeezer clones: The Orange Squeezer, designed by Dan Armstrong, is a classic vintage compressor which has a cult following among guitarists. There are many, many modern OS clones out there, such as the Analogman Juicer or Pedal Doctor Tangerine Squeeze; and a few interesting variations on it such as the Toadworks Mr. Squishy.
The OS is so popular to clone because it's a very simple circuit, and it has a distinct compression action and tone which are unique and appealing on guitar. It can add an emphasis to the leading edge of the attack of your notes, quickly followed by a little cut of the signal level, which rises back up in a blooming swell of sustain. I describe the action as a "dip and swell", and guitarists often refer to a "chicken pickin" country sound. The guitar solo in the Dire Straits song Sultans of Swing is a classic example.
The original OS only had an internal "level" trim pot, but it is also possible to get a wide range of compression action, from subtle to extremely squishy, by adjusting the bias of the transistors. For this reason, some modern OS clones include a "bias" knob; I highly recommend this feature, as it allows you to get more variety from this effect, although some builders prefer to find the one "best" setting and fix it there.
The strong dynamic reduction of the OS brings forward the higher harmonics of your tone, and sometimes adds a bit of light distortion. It tends to enhance noise as well, but the amount of noise can depend on how a specific pedal was built. The original OS design rolls off some of the low end, but there are some relatively simple mods which can be done to it to allow better bass frequencies. Check out tonepad.com for more info about those mods. Unfortunately even after modding for bass, an OS will never have a deep, fat low end sound, because the intense compression (the same effect that gives so much sustain and swell) takes away the perceived depth or strength of the lows, even though they may not actually be rolled off.
For those of you handy with a soldering iron, a simple OS kit and plans are available from generalguitargadgets.com and other sites to build your own fairly cheaply. Note that different component choices can really change the sound quality of this circuit, so be sure to experiment with different opamps, etc. Some of the cloners who have "improved" the design seem to have tried to engineer out its unique qualities and instead emphasized the more boring smooth/transparent compression it is capable of. I guess there's nothing inherently wrong with that, but in my opinion the beauty of the OS is in its distinct personality and eccentrities; so I'd encourage you to buy an OS for its uniqueness, and buy something else if you want transparency.