How tight is the remaining bolt in the manifold? I've used electric engravers to back out broken bolts/studs. The vibration loosens things up to a point. Just do it in a counter clockwise rotation. This may work. Some times a using a center punch tapping it works also. One last desperate thing to try is welding a bolt or a nut on to what's remaining of the old bolt. That way you have something to grab.
I have used easy out's from Craftsman for bolts as small as M6. You should be able to use an easy out no problem, the one you broke is probably M8. The left handed drill bit mentioned previously works a good deal of the time as well! Just make sure your drill is in reverse.