What's stripped, the head or the threads? If the head is stripped, you can try dremeling or filling a slot in the head for a flat blade screwdriver, but if it's on tight you won't be able to apply enough torque before the slot strips out. Other than that (and assuming this bolt goes into a blind hole - ie. there isn't a nut on the other side you can cut off), there's only reallly two options unless you have access to a welder or cutting torch:
1. Try using a bolt or screw extractor. They're not expensive and come in different sizes. They have a cutting tip that is left hand threaded, to they bite in as you turn them counter clockwise. Their biting action digs into the head of the bolt, and since they're reverse threaded, as they bite harder they transfer more torque to the bolt in the direction that unthreads the bolt.
2. Drill out the head of the bolt. You'll need a drill bit that's at least the same diameter as the threaded portion of the bolt, and enough clearance to use a drill bit and drill. Drill in the center of the bolt head, and the head will shear off once you get in far enough.
If you don't have enough clearance to do either of the above, you could try welding a socket onto the head of the bolt, or, use a cutting torch to cut off the head of the bolt. This requires a bit of care though, to avoid damaging other parts nearby.
For things like head bolts, do not even waste your time with extractors, etc. You will likely wind up with an even bigger mess on your hands when the extractor breaks or you have to tap new threads and you have a crooked bolt.
If you can get hold of a MIG welder just place a nut over the broken bolt. You want to weld the inside of the nut so that you are essentially attaching it to the broken bolt. If you are doing this on a cast iron block there is no need to worry about welding the nut to the block, it ain't gonna happen.
Usually the heat from the MIG will loosen up the bolt so that it easily turns out with even an old pair of pliers.
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