Here's a must-buy for all you NYC-architecture nerds: a manhole doormat made out of recycled tires! FLAXart has models for New York City, London, and Paris-- so if you've got friends overseas, this might make a nice gift. You can also buy the NYC version at the NYC.gov store, if you'd like to give the money to the city.
If you're not sold yet, check out these manhole links:
Forgotten NY's Manhole Page
NYCsewer.com-- manhole paintings!
NYC Manhole Group on Flickr
Tattoo by Manhole
Wow-- that's a lot of manhole!