So, seeing someone comment on the Alternity system in another thread, I thought of something:
Many gamers are collectors, who will seek out obscure systems, and spend lots of energy tracking down
out-of-print systems.
For me, my best find, that took me 7 years to track down was: Stalking the Night Fantastic, the Bureau 13 RPG.
Systems that I am considering in my Holy Grails to find: TSR's the Amazing Engine system and TSR/WotC's Alternity
system.