A non-exhaustive list of online resources to monitor weather and climate.
Global climate
NOAA PSL Weather/Climate Plotting and Analysis Tools
National Snow and Ice Data Center (NSIDC)
Climate variability & teleconnections
CPC Teleconnections (NAO, PNA, etc.)
UK climate
Met Office UK climate maps and data
Met Office surface analysis archive
Tornado and Storm Research Organisation (TORRO)
Stratosphere
NASA GEOS stratospheric seasonal evolution
Weather observations
NOAA National Data Buoy Center
UK & Ireland radar (Netweather)
Starlings Roost Weather (Dan Harris)
University of Wyoming Upper Air (radiosonde) archive