There are about three tachometers in wide use for helicopters.
The first is the Anderson Hobby tachometer. This is fairly cheap about $35) and works well on the ECO/Logo, but it does NOT work on micro helis such as the Piccolo CP! It also drains batteries quickly even when not turned on, so I recommend removing the battery when not in use.
The second is the X-cell optical tachometer. This is expensive(about $150) but works with almost any helicopter. It only reads to 1990 rpm, so it may not be suitable for micro helicopters which typically need 2000+ rpm of headspeed.
The third is the Tera tachometer distributed by Thunder Tiger. This is a very nice digital tachometer specifically for helicopters which will clip onto the tail boom for hands-free measuring of the headspeed.
There are other tachometers, including:
Magnum mini-tach. Price is about $35.
Hangar 9 micro digital tach
Thunder Tiger 2642 mini tachometer
For some reason, Quantum Models www.quantummodels.com stocks more tachometers than any other online store I've seen.