National Radio Societies:
RSGB - The Radio Society of Great Britain
ARRL - The American Radio Relay League

Contest information:
HFCC - The RSGB HF Contest Committee
VHFCC - The RSGB VHF Contest Committee
SM3CER - A compendium of worldwide HF contest information

Propagation:
MUF maps - real time world wide MUF maps
European - real time European ionospheric maps
Data - lots of real time propagation data

Band activity:
DX Cluster - real time and archieve dx cluster spots

DX Licence data:
Country info - worldwide licencing info
Call search - world wide on-line call book

Useful general links:
G7KPF - amateur radio links