Over at TechCrunch, Frank Gruber just posted a brief comparison of mapping services from Ask, Google, MapQuest, Windows Live and Yahoo.
Mapquest is the most popular mapping service but lags on features and usability. Google is the most notable and has a ubiquitous API. Windows Live Local dazzles with its creative views and features but falls short of the others in direction functionality. Mapquest offers a number of features but still is missing satellite imagery, which makes it trail the competitors in core functionality. Ask Maps is a worthy competitor but had the highest error rate of the group.
Overall, Yahoo Maps was by far the best application tested. Its fast Flash interface, multipoint directions, live traffic information, and easy send-to-mobile feature make it the hands down winner. It also features the most robust API options.