I am not sure where this belongs. OP, feel free to move it!
Desko PBT RGB Dye Subs (this is almost always on the list)
Cherry PBT Dye Subs
356 series
Original KMAC (KMAC LE is not as rare, since I was able to distribute them)
Click Clack's?
Cherry Red Esc Double shot (for awhile)
Green switches
Grey switches
White switches
I can't think of anything else.
LZSE's
SoWare
LZ-F
Gold Plate Springs
Korean Springs
HASR0
Phantom
Model M15