When Google Maps first appeared on the scene, it wasn't a big game-changer in the online-map market. Mapquest was pretty well-established, and Google didn't really seem to have much to offer that Mapquest didn't have, apart from the ability to click and drag the map around (hey, in 2005 that was a big deal). But over time, Google Maps has become less about just maps and has evolved into a visual interface for all kinds of information, a sort of two-dimensional version of Google Earth, the company's 3D satellite-imagery service -- especially with the ever-expanding "Streetview" feature, which shows actual photos of the location you're search for (not available in Canada yet).
It started small, with features such as the ability to integrate Google Maps and the GPS in your phone or BlackBerry -- a feature that has saved me from getting lost many times, and effectively turns your phone into the equivalent of one of those dashboard GPS units, at a fraction of the price. Then Google started allowing you to add "user-generated content," such as your own locations and custom maps (something which has helped the company map remote parts of countries like India). You can do things like clicking two points and Google will calculate the distance between them, or list historical monuments in a certain area.
Now, Google has added some new features that expand the service even further: one is geo-tagged photos from Google-owned photo services Picasa and Panoramio. If you click to turn on the feature, the map you're looking at starts to sprout thumbnail pictures at various spots. It's like a low-rent version of Streetview, with the photos supplied by users instead of being taken by a 360-degree camera mounted on a Volkswagen Beetle. And it can add a surprising amount of colour to your search, especially if you're not familiar with the place you're going (of course, you might have to put up with some photos of people's kids and dogs as well).
You can also turn Google Maps into a real-estate search tool, which allows you to search by location and price, and then sort the results until you find what you want (services such as Housing123.com effectively do the same thing by "mashing up" an MLS feed and a Google Map). And finally, Google has added Wikipedia entries for various locations -- so that when you search Halifax, for example, you can click on the thumbnail of an encyclopedia entry for the city and read about the famous explosion in 1917 that destroyed much of the town.