It may not be the handiest tool, but I've made good use of my ViceGrip chain wrench. Unlike some clamping wrenches, like a pipe wrench, the chain wrench grabs a round object all the way around, without teeth that can mar the part. I've used it on headsets when my confidence in the wee key on the washers is very low (most of the time). On small parts you need to think ahead on how to keep the excess chain out of the way.

I've recently seen a vintage VAR headset wrench, sort of like the wrench nlerner showed above. Nice tool, that.