Don't Lose Your Mind for Don't Rest Your Head
Wolsung: Steam Pulp Fantasy
Cthulhutech Quick Start Rules
Cthulhutech GM's screen
Cthulhutech Core book - I like having multiple copies, so I have one and my players can also reference one.
Vade Mecum for Cthulhutech - it's basically a core book, honestly. It's essential for play.
Dark Harvest: The Legacy of Frankenstein - Resistance
Laundry Files: Agent's Handbook
Clockwork and Chivalry Core Rulebook (The 2nd Edition which uses its own system)
Marvels of Science and Steampunk for Victoriana
Tales from Wilderland for The One Ring (which is amazing)
Loremaster's Screen and Lake-Town Sourcebook for The One Ring
Two sets of the custom dice for the Mistborn RPG, including the promo 11th die.