There are many excellent websites for the bus enthusiast but I especially mention the following ones that I find most useful:

The PSV Circle :http://psv-circle.org.uk/

The Omnibus Society: http://omnibus-society.org/

Buses Around the World by Stuart Bell :http://stuarts-world-buses.smugmug.com/

Oxford & Chiltern Bus Page: http://www.oxford-chiltern-bus-page.co.uk/

The Bus Station: http://busstation.net