Maps
Gawker has a nice list of NYC maps. The most useful ones are probably the Citidex maps for neighborhoods, parks, stadium seating plans, ZIP codes, police precincts and so on. Skyscraper.org has a nifty interactive set of layered maps, showing historical features.
Additionally, NY Site’s maps are also interesting for the Upper West Side, since they show what’s actually on each block, in terms of stores, residences and lot numbers.
Sadly, I found the Gawker site through recent Google searches that came to my site, where the searchers were looking for the old, very detailed and useful MTA neighborhood maps. The MTA, for unknown reasons, have taken the links down. We’ll have to venture into the subway stations to see these now. The Citidex street maps are detailed, but they don’t show subway entrances or platform alignments.