what nick said is sorta correct, i was talking about the Shure SM57 and SM58, they are world wide used microphones, a lot of them in metal albums, from Metallica to the Swedish Death metal scene.
The SM57 is most used on guitars (but not exclusively) because the frequency response as nick said
that "bump" at ~8k makes the sound sharper, giving the guitar a nice sound, but it doest mean it's just for guitars:
[ame]http://www.youtube.com/watch?v=LMoOrrKl3k4&feature=related[/ame]
The Shure SM58 is used a lot for vocals, because because it's frequency range has a bit more of low end, despite looking very similar, it makes a diference
most people use the 58 because it gives a "warm" sound, good for metal music
also, they are around ~100 dollars and they are build like a tank, which makes a great stage mic