The Viele Map of Manhattan was made in 1865 and shows the original boundaries and waterways of the city. Here's a thumbnail view (with prints for sale) and the David Rumsey Map Collection has a zoomable version that you can explore. (thx, meg)
Update: Took me forever this morning, but I cobbled together a high-res version of the Viele map from the PITA Java applet on the Rumsey site. Warning: the image is quite large (9859 x 3115, 8.6 Mb) so it might crash your browser if you attempt to look at it...better to save it to a local drive and open it up in an image viewer.
Update: Here's a simple zoomable/scrollable version (a la Google Maps) of the high-res image that I whipped up with Zoomify. Thanks to Aaron for the suggestion.