First, there was MapQuest. In 1998, if you wanted a map or directions, it was the website to visit. Online maps were fairly small images, the zooming in and out feature of the map was time consuming as you waited for the page to refresh, but it was neat to get directions from point A to point B, even if they did put you through a rough neighbourhood or take some convoluted route.
Then in February of 2005, along came Google Maps. And while providing similar functionality to MapQuest and even the same data (both use Tele Atlas & Navteq map data), they really took things a step further with their dragging features, snappier zoom, and their dual satellite/maps view. They focused on a superior user experience that won me over in the first five minutes of using it.
While the user experience was compelling, probably the most significant feature of Google Maps was their programming API. In the summer of 2005, Google Maps released its programming specification to the general public to use for free, allowing developers to build on top of their mapping platform. Now you too could incorporate Google Maps and all of its functionality into your web application. Functionality previously reserved for high-end GIS systems was now in the hands of programmers everywhere. This open API diffused Google Maps innovation rapidly across the web, using the smart ideas and developer know-how of thousands of people around the world to build all sorts of amazing applications.
The most famous of these so-called web application mash-ups was the combination of Craigslist data and Google Maps at HousingMaps.com, combining houses and apartment rental data with maps. A simple and brilliant idea, made possible by combining two open-API technologies and creating something new as a result.
If you've never played around with Google Maps outside of just searching for an address, there are many sites where you can see how people are using mapping technology creatively for their business or just for fun. The website Mapki keeps tracks of quite a few of the Google Maps projects, ranging from map visualizations of crime information to a pub-crawl planner of the UK to real-time weather and webcam data of North American cities.
You can even create your own custom maps (an interesting tourist map for your relatives coming to visit, for example) over a Quikmaps.com. They've built a map-making tool on top of Google's Maps API. Or play around with geocoding your pictures and browsing the collections of others at loc.alize.us, a Flickr/Google Maps combo. LifeHack.org has a good list of essential Google Maps implementations as well, including some links to sites that will help you get started building your own Google Maps application.
As developers of web applications, it's exciting to see how easy it is to integrate third party functionality for free. However, with no guarantee of service it is important to avoid becoming dependant on these services and to always ensure that a contingency plan is in place if the service is dropped or is no longer suitable.