New York Songlines: NY Songlines gives a great block-by-block tour of points of historical interest all across the city.

Bowery Boys: These guys do an awesome podcast focusing on a wide variety of events, people and places in the city's history.

New York City Walk: This guy did it first (finished in 2004).

How I Walked All of Manhattan in 10 Weeks: This guy did it fastest.

I'm Just Walkin': This guy did it all.

WalkSafeNYC: Be safe out there! (a reader's contribution)