The Kawartha Branch of the Ontario Genealogical Society serves Northumberland County (as well as Victoria, Peterborough and Haliburton). It is based in the city of Peterborough. Its website includes a series of photographs of the places of worship in the area, and a publications list (currently all publications are on paper)
At the time of writing (Nov 2012) Northumberland GenWeb advises that it is looking for someone local to update their website and all links revert back to OntarioGenWeb's general advice.
Cobourg History is a website providing background information on the largest urban settlement in the county. Cobourg was founded in 1780. Its harbour has always made it quite important.