Trulia (makers of many interesting web maps), have recently come out with the commute times in addition to their maps on property values, crime, school locations and amenities. While the algorithm is not explicitly mentioned in the map, the tool does allow for a differentiation between auto and transit modes.
Travel times are calculated based on the mode, and displayed as an isopleth map (kind of like contours, but with colours). It seems like there is a straight-line distance from certain points (such as transit stops) that are overlayed transparently to give the image of a raster-like surface. The tool is based on Google Maps and can be changed interactively depending on the user’s maximum allowable travel time and preferred mode.
(via Google Maps Mania)