I added some children, previously missed as they were born and died between censuses, to my family tree yesterday thanks to a database of burials at Abney Park Cemetery in London.
Abney Park, 32 acres in north-east London, catered to Non-Conformists (Methodist, Baptist, Congregationalists, Wesleyan etc.) Burials start in May 1840.
There is a free basic search available without registration. Free registration gives you access to an advanced search where you can limit the search to specified years and/or section of the cemetery. There is also a "sounds like" metaphone search and the ability to screen an entire GEDCOM.
An earlier London burial ground for Dissenters was Bunhill Fields which is indexed as part of a collection of City of London burials by findmypast.com.