For me, essential websites
weather:
Avbrief
http://www.avbrief.com - well worth paying for the data there
GFS
http://www.arl.noaa.gov/ready/cmet.html - under "WMO code" enter the airfield ICAO code (small airfields not supported), click on Continue then choose e.g. the 0-84hr GSF model, etc.
Notams
AIS
http://www.ais.org.uk
There's a load of other weather sites e.g.
http://www.sat.dundee.ac.uk/