If you live there or are planning to visit New York City this online subway map hacked onto Google Maps might be useful. For the rest it is another interesting application that might come to your city soon.
Via Metafilter.
Update: From Kim in the comments:
www.hopstop.com is the absolute best subway tool I’ve found. Door to door directions in 3 cities, including buses, subways, and walking.
I gave it a quick look and Kim is right. If you are in New York City, Boston, or Washington, DC. this is a great tool! I found it easier to use than the onNYTurf site linked above and though HopStop doesn’t give you a map of the subway system it quickly gives what appear to be excellent and detailed directions.
That’s a very cool tool. http://www.hopstop.com is the absolute best subway tool I’ve found. Door to door directions in 3 cities, including buses, subways, and walking.
I just checked HopStop out using two old addresses. The directions will get you there, but in a long and time consuming way. Certainly wasn’t the most direct route.