Power Cordz has just released their new 1.2mm derailleur cables weighing in under 3g each for normal length applications.  A typical steel cable weighs 12g to 17g, making this at least 75% lighter.  For a full bike kit (front/rear brakes and derailleurs) the savings can add up to an ounce or more.
Power Cordz has offered 1.5mm derailleur cables for a while, but some reviews and comments complain that they have a hard time fitting them into normal housings or fittings.  The new, slimmer diameter aims to solve that problem and is compatible with SRAM, Shimano and Campagnola shifters.
The cables are made of PBO fibers (about 9,000 to 10,000 of them per cable) encased in a nylon coating to reduce friction.  They claim to last up to three years, and you can get them in full sets with housings, just the cables or cables plus a Nokon-specific liner if you use Nokon housings.
