Dynaudio's OCOS Speaker Cable
Dynaudio's Mike Manousselis introduced me to the OCOS (Optimal COnnection System) coaxial, copper speaker cables ($500/3m pair). OCOS insists that a speaker cable should be completely transparent, imposing no sonic signature of its own. Manousselis explained that the cable provides a constant impedance throughout the entire frequency range, so as to stabilize the partnering amplifier, allowing it to "see" the speaker directly.
I like them because they're skinny and not crazy-expensive.