disk cutter ?
i'v yet to find a decent broken stud remover and rounded nut undoer yet but the most usefull tool i have is long/deep sockets! got some in half inch but looking for a set of 1/4 3/8ths drive.
another great "tool" is those boxed allen and torx straight err keys with the sockets to fit your socket handle you can put more welly on them than the bent ones.
the set of allen keys mentioned earlier are great but saddly missing for some strange reason 7mm which my golf has aplenty ,i cannot find a single 7mm of the same type to add to the set to make up the deficiancy