The geniuses at Google Labs have been busy ... here's a neat website for those who don't have a car: Google Transit, which aims to tell you how to travel using only public transportation.
Right now, it is only available in 12 US cities (but the entire country of Japan. Huh!): Link - via Intelligent Travel, Thanks Marilyn Terrell!
or at
http://tripplanner.metrokc.gov/cgi-bin/itin_page.pl?resptype=U
I'm sure it will be a nice feature at some point, but for now there are better solutions.