Derailer - Rear - Shimano Deore (model M591), black
The Toyota Camry of wide-range rear derailers. Shifts to 36t. For 9 speed cassettes.
Shimano code name RD M591 SGS, in which RD = rear derailer; M=Mountain bike series; 591 = the numerical grade in the shimano numerican hierachy that used to go to 7XX, but now it's up the 9XX.
---------Feb 4, 2020 NOTE:
These were available in silver but Shimano decided to stop making those - drat... These black M591 derailleurs will also go away forever, supposedly sometime later this year, unfortunately.