For some of my more 'green-conscious' clients, or those who wanted a more active lifestyle, I have recommend the Walk Score website. This website gives you a walkability score for any address that you input. They base this off of how many parks, schools, theatres, grocery stores, businesses, etc are nearby. They don't take into consideration if there are sidewalks, safety, weather of the area, topography, etc but it's a good start for those who are considering moving to a new area and wondering what the day to day activeness would be like.
For example, one of my recent "Pick of the Week" properties showed up like this after inputting the address:

It gave this Irvington home at "very walkable" score of 71, not bad! If you're lucky and live downtown near Mass Ave, you'll get a high score of "Walker's Paradise". Sounds nice, right?
On their website, they also have info about public transit, rankings of neighborhoods in cities, and more. There's even an iPhone app! Just another handy tool to help keep you informed while making an important buying decision.