just about every company makes a lighter core for some of their boards, but generally they do it for only the pro model boards and thus the price can go up pretty steap, as high as $700 for some.
CWB --> "platinum" core
Hyperlite --> "DINE" and "NOVA" cores
RONIX --> DARK CORE
LF --> next year is introducing the "FLY" core
GATOR --> has a machined PVC core, forget the name, but i think only one board has it
SLINGSHOT --> all their boards are a hybrid of wood and closed-cell foam that is really thin and light... not THE lightest, but very light nonetheless, especially in "swing weight"
I might be missing some companies...?
sometimes the way the board is shaped will help it "feel" lighter, though the overal weight is the same. you can look for boards with a dished out tip and tail. this will concentrate the weight under the feet and make the "swing weight" less. a good example is (but not limmited to) hyperlite tribute --> heres a vid of the tribute that also describes the "swing weight" (http://www.youtube.com/watch?v=zV3gfPh-fT8
). you can find other videos of hyperlite boards and other companies boards on youtube too