Weather Data

Radar

UK Met Office - 30 minute updates, UK only
Netweather - 5 minute updates, and zoomable UK map
Meteox - 15 minute updates, also covers Europe

Sat24 - Visible and IR loops covering Europe, 15 minute updates
UK Met Office IR Loop - regular updates
Meteosat - Regular updates

WWLLN - Worldwide Lightning Location Network
Icelandic Meteorological Office - Covers UK and NE Atlantic, uses ATD data (accurate)
XC Weather - UK and Europe surface data, updated every few mins
UK Met Office - UK observations, hourly
Meteocentre - UK surface analysis, hourly

Weather Online - UK maps, all models including GFS
Wetter Zentrale - Comprehensive model data and maps, in German
Net Weather - Model output maps

UK Met Office - Comprehensive UK and international forecasts
Weather Underground - 7 day international forecasts
ESTOFEX - European Severe Weather Forecasts

Stormtrack - US severe storm forums (recommended)
Chasseurs Orages - French storm chasing forum
Weather Zone - Australian forum, good photography tips