A Swiss Vetterli, converted from 41 rimfire to 41 centerfire makes for a fun (and different) shooter. Also loading for a 50/70 trapdoor, 7.62 Nagant revolver a Lee Enfield 410 musket (brass case, not rechambered for modern 410's) and a slew of old pocket pistols for which ammo, when available, is simply more expensive than I want to pay.
I also scout the shows for those I can't load such as 32 and 41 short rimfire, 25 Stevens and 22 Winchester auto. Some of those have been reproduced in recent years. I've considered buying the brass cases that use a 22 blank for ignition for the 41 and 32 though I'm not sure how well they would work in the revolvers.
Over the years I've gotten more than a few oddballs, for me part of the fun is getting them out shooting again.