Walk Score

Here is a nice little tool to compare the walkability of an address: www.walkscore.com
The tool uses a simple weighted scoring of distance to amenities such as shops, restaurants, schools, banking etc. to rank how walkable the address is. They have also produced a walkability Heat Map and ranked many US cities.

The tool is fun to play with but has some major limitations that makes comparing addresses not always that useful. Some of these shortfalls that may give strange results are:

• walk-score does not appear to include distance to transit and its frequency in its calculations. It has a separate transit accessibility score (where transit agencies make data publicly available) but this is not included in the walk score. I believe this is a major flaw as the ability to access public transport to employment and major service hubs is a large contributing factor to improving the available amenities within 'walking' distance. Although you may use public transport it does reduce car dependence. 
• Walk score does not include distance to employment opportunities.
• The application is dependent on information being available on the internet. Because of this the city-edge apartment I owned in Pietermaritzburg scored a lower Walk Score that my current mid-suburban home in Brisbane - because most of the amenities within walking distance of my previous apartment are not captured on the web.
• the assessment does not take into account what facilities are available for pedestrians. An address along a major road with no pedestrian facilities would score the same as one within similar distances to amenities but with pathways and crossing facilities.