[Smaug123]

I use Major for dates, and I know the periodic table as a mapping of number <-> element <-> symbol. (And, by accident, I know my Oyster card's serial number.) It also is handy if you want to carry a number around briefly without writing it down - I've used it for carrying numbers from computer to computer at work. With lots more practice I could probably get near-real-time memorisation, and the possibilities would expand much more.