At the more pricey end is the Schertler DYN-V. A friend has one for his guitar, which I tried out on my fiddle. It consists of a small dynamic microphone which attaches face down temporarily to the instrument's surface with a circular bead of putty (like silly-putty). You can find the best spot by moving it around to different areas. The sound is a very true (pure) acoustic sound (unlike most of the piezo transducers). I was convinced and ordered one ($410 over the internet), which I will use on my fiddle and also on my guitar. It plugs into a XLR mic input on the sound board and requires no additional preamp, other than what is on the board (acts just like a regular dynamic mic).

Cheers, S. in Seattle