Lost New York City is Randall Dana's website, which documents 19th century buildings that were destroyed in the 1970s. Mr. Dana is a sculptor who creates architectural elements of the 1800s such as hand-cast gargoyles.
LTV Squad document things in a different way. They are "a group of professionals, historians, artists and infrastructure fanatics that are in love with all things abandoned or not commonly seen by the public." Mostly that means 'forbidden places.'
Whitman's Brooklyn is a delight: a 'virtual visit circa 1850' enabled by drawings, paintings, and maps of the borough. Highly recommended.
Bowery Boys 'A weekly romp down the back alleys of New York City history,' which includes weekly podcasts. For history buffs or anyone who just loves the city, a must-read (or listen).
Colonnade Row Wide-ranging musings on those landmark buildings, and the neighborhood past and present.
Old Kew Gardens The story of that section of Queens, with lots of great then/now photo pairings.