Zalman CLPS-7500ALCu series (a mix of copper and aluminium) there are several models based on fan size (92mm - 120mm), so get a 92MM CU-AL, as it will be very good, as well as light enough not to tear of the socket brackets off your motherboard, and the open structure design cool your north bridge too. a well recommended design.
ADDITION: just been to their website and seen a newer model CNPS8700 NT, is 25gm less in weight, and i cannot believe its pure copper, as copper is tend to be heavy, may be an alloy. but is more promising.
ANOTHER ADDITION: same manufacturer different model "CNPS9500A LED", this comes out to be far most better at 530gms and you wont be requiring a case fan. as it will throw out CPU's heat plus systems's heat out with one fan.