my bad, its called a "power extractor" search for that. basicaly its a reverse drill bit that you drill into the broken bolt/stud and when you get several mm in it bites in and unscrews.
If the bolt/stud is above the surface enough so you can put a tool on it they have a bolt/nut remover too.
use penetrating oil, heat and try and hit the bolt to losen it up before you do anything though, if its REALY stuck you might as well drill it out.
http://www.arizonatools.com/ca...=1010
http://www.merkurencyclopedia.....html
http://www.brokentap.com/easy-outs.html