Can someone run down for me the various dropout spacing standards (120, 130, etc), talking about what kind of bikes you find them on, and what type of parts are readily available in those sizes?

Also, do Shimano Nexus hubs come in all widths, or only one? Is there such a thing as a Nexus hub that is disk brake compatible? I want to see how many different oddball technologies I can (mentally) build into a money-no-problem four-season commuter.