One on my favourite live sound instrument mics now is the Behringer C2. The sound guys put one on my cello when I was playing for a ceilidh with English Rebellion at Shrewsbury Folk Festival. I'd never seen one before, and it worked brilliantly. I bought a pair when we got home after the festival.

The best part: it's a real small diaphragm condenser mic for about £40 a pair complete with stereo bar, pop shields, clips and a smart tough plastic carrying case. It has a pad/bass cut switch on it too. Not in the league of Schoeps or Neumann of course but at that price you can afford to drop one in the mud and still have change left from the gig, and to carry a few spares. And it sounds OK - certainly no worse than a SM57 especially on the more delicate end of the acoustic instruments scale that work better on condenser mics. I've used them with all my squeezeboxes and cello. It should be fine on a guitar or fiddle too.

I have better mics for recording with, but they rarely come out of the studio...

Anahata