Parish registers online
1.2 million Durham baptisms and marriages are included in the IGI at FamilySearch but not all parishes are included (you can see the coverage here). FamilySearch also has browseable (but unindexed) images of Bishop's Transcripts and Marriage Bonds.
Findmypast also has transcriptions of 156,000 burial entries from the National Burial Index.
The volunteers of FreeREG have transcribed some of the registers for about 240 Durham parishes..
Births, marriages and deaths
Registration districts in Durham