These Zener diodes can drop ~6V each, at up to ~1.85A (3W power dissipation each). Two can be wired in series to drop 6V, and this is remarkably effective at ‘converting’ 12V fans to operate across the full range of 24V PWM input.
http://www.mouser.com/Search/ProductDetail.aspx?R=1N5919BGvirtualkey58410000virtualkey863-1N5919BG
Mouse Part Number: 863-1N5919BG