Portland, Oregon. I haven't owned a car for 3 years, and I had another 2 years car-free a few years before that. The closer you are to downtown, the easier it is. There's good public transit and ZipCar has a big presence here, in part because it bought local business FlexCar, which was one of the first carsharing companies in the US. There's also a strong bicycle presence which makes for (relatively) educated car and bus drivers, good signage, and many bike lanes.
The only thing that's missing is a major graduate school, or I wouldn't be moving away! Portland State University has some strong departments but poor funding overall, especially for graduate students. Other than a few professional schools, there aren't other options for fulltime, in-person graduate study here. Oregon State University and the University of Oregon are 1.5 and 2 hours away, by car. Unfortunately, public transit doesn't extend that far (yet), and neither Amtrak nor Greyhound are ideal for commuting purposes.
hhw got a reaction from TheUnbearableLightness in best US cities without a car