Just Xolos (2), Kooikerhondje (1) and Mudi (7) I think. A Fila and a Boerboel too. Oh, and a Neapolitan Mastiff. A handful of Tibetan Spaniels. Coolies (3) also, but for some reason I don't think of them as being uncommon, but I suppose they are. Probably just because online they don't seem uncommon at all.
I also met a Norwegian Lundehund once at a dog show walking around and I knew a Norwegian Buhund.
Other than that, I can't think of any other breeds I've encountered that are considered rare. I know 3 Ridgebacks and have been around several others... so I don't consider them uncommon, but maybe?
There are also other breeds that I've seen around that are uncommon, but have not actually interacted with/met them.