Weather Data

Radar

UK Met Office - 30 minute updates
Meteox - 15 minute updates, also covers Europe
MeteoGroup - 15 minute updates, including Europe

Satellite

Sat24 - Visible and IR loops covering Europe, 15 minute updates
UK Met Office IR Loop - 1 hour updates
Meteosat 8 Visible - 3 hour updates
SatRep Images - Updates vary

Sferics (Lightning)

WWLLN - Worldwide Lightning Location Network
Icelandic Meteorological Office - Covers UK and NE Atlantic, uses ATD data (accurate)

Current / Recent Readings

XC Weather - UK and Europe surface data, updated every few mins
UK Met Office - UK observations, hourly
Meteocentre - UK surface analysis, hourly

Model Data

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

Weather Forecasts

UK Met Office - Comprehensive UK and international forecasts
BBC Weather - BBC UK and international forecasts
World Weather - International forecasts and climate info
Weather Underground - 7 day international forecasts
ESTOFEX - European Severe Weather Forecasts

Weather Forums

UK

UK Weatherworld - UK forum
The Weather Outlook - Chat and forums, including non-weather

International

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