I too have broken a water pump bolt off, but I was able to drill it out and use a helicoil. But I have also broken off a easy out on a friends carburetor mounting bolts on his land cruiser and I used a technique similar to ledfoot's. I used a center punch to push around the easy out until it widened the hole that it was stuck in. The bolt is much softer so will deform while you bang around on the easy out piece. Eventually I was able to loosen it up enough to get it out. Then you can drill it and helicoil it unless you know someone with a welder.