The first major online maps site to hit it big was MapQuest. And they are still the industry leader, especially for driving directions. However, the big 3 Internet companies--Google, Microsoft, and Yahoo--are charging hard with their own maps, all of which also include aerial images.
The market having established itself, TechCrunch has pulled together a thorough review of the maps sites available. Weighing all factors they have chosen a favorite. Note, this is only as it stands right now-- these things change seemingly week to week, as seen with Google Maps' major updates at the beginning of this week.
But, as I already had intimated at the right, Yahoo is the current leader. Says TechCrunch,
SummaryAll the sites have some pretty cool features if you haven't visited in awhile. I'd encourage you to check them out. The future of mapping is very bright as novel ideas continue to develop.
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.