I think many of us have heard that 'Antonovka' is quite true to seed, and is often used as a rootstock.
I've run into mentions of various other cultivars that supposedly exhibit an unusual degree of true-to-seedfulness, mostly in the context of providing good rootstock for certain conditions.
The ones I've heard of:
Antonovka; the obvious culprit, pretty easy to find online. I've got some seedlings just coming up now. Whether they're actually Antonovka is another question, time may tell...
Beautiful Arcade seedlings; apparently commonly used as rootstock in the maritimes, but I haven't spotted any seeds sources yet.
http://www.appleman.ca/korchard/rootstok.htm
Borowinka, another Russian cultivar, supposedly tart but edible and often used as hardy rootstock.
For sale here and there:
http://www.seedman.com/fruit.htm,
https://sheffields.com/seeds-for-sale/Malus/pumila////Borowinka///877//Borowinka-Common-Apple
Fedco Seeds states that 'Duchess of Oldenberg' seeds are called Borowinka and used as rootstock:
http://www.fedcoseeds.com/trees/?item=123
Fameuse, AKA Snow, mentioned here:
http://skillcult.com/blog/2013/04/03/apple-breeding-part-1-everyone-knows-you-cant-do-it-right
This one at least I do not need to look far for seeds, as we've got a Snow in my parents old orchard. Too soft as an eating apple IMO, but very sweet and a nice addition to juice/applesauce. I'll save some seeds this summer.
I'm interested in hearing about other varieties with this reputation, as well as more information about any of those listed, and sources for seeds. So, what do y'all know?