I Google Maps. Of course, like everything Google, the Maps site is constantly improving and adding new features. On top of that, Google Maps is a phenomenally useful resource for creating mashups.* There is a blog called Google Maps Mania that tracks the websites, mashups and tools being influenced by Google Maps. Today the blog is reporting on a new feature of Google maps called "Draggable Driving Directions" that allows you to easily make changes in a route that Google maps plans for you (includes a brief video). The Google Maps Mania blog also has a listing of lots of other useful mashups and tools - check it out.
*A mash-up is a Web page or application that combines and integrates the content of two or more sources to create something new. For example, there's a zip code map mashup using Google Maps.