Yahoo has added some mapping features to Yahoo Travel, using AJAX to add layered images right on a Yahoo Map. For example, a map of the Boston Freedom Trail can be overlayed on a map of the city. Or a map of the Philadelphia International Airport can be matched to the highway map.
The opacity of the layers can be adjusted, so a user can choose to see one or the other map, or a mix of the two.
Greg Sterling has more details at his Screenwerk blog, and says that he expects the functionality to be extended to other parts of Yahoo, including local search, in the near future.