... to save argument, in alphabetical order within each group!

Model Bus Federation As far as I know, the only organisation devoted to the world of model PSVs.
Other Model Sites:
Crossways Ryan Birch's Gloucestershire-based fleet
London & Country Buses Alan Graves' model fleet
Padbus Paul Dudfield's Padbus Website
Redrover Buses The Red Rover model fleet
General Bus Sites:
Bus Station (The) A list of over 1300 sites, many of which are in the UK.
EFE Exclusive First Editions
Johnsons Johnsons Coaches of Henley-in-Arden
Public Transport Information More general travel information
Redwing Coaches Official pages of this central London firm
Scania Scania's Official Website in Sweden
Showbus All kinds of things - even buses for sale.
St. Petersburg Tram Collection Absolutely fascinating collection of 1:43 models
Major Groups:  
Arriva : FirstGroup : Go-Ahead : Stagecoach Holdings