Step 1: Sign up for a Google Maps API key via this URL.

The Google Maps API lets developers embed Google Maps in their own web pages with JavaScript. You can add overlays to the map (including markers and polylines) and display shadowed “info windows”.

A single Maps API key is valid for a single “directory” on your web server, so if you sign up for the URL http://www.mygooglemapssite.com/mysite, the key you get will be good for all URLs in the http://www.mygooglemapssite.com/mysite/ directory.

Step 2: Upload a minimal HTML page provided by Google Maps to your web server. This URL shows you what I got from Google Maps.

Step 3: Study this tutorial: Google Maps EZ — Get Started.

Step 4: Follow the given examples in the tutorial and change some code to suit your needs.

So, building a location-based web service using Google Maps is not so difficult as you might expect. What you need is a good idea. This Wiki Web, Map Projects - GoogleMapki, does a good job in collecting information about what people are doing with Google Maps. It also demonstrates the usefulness of Wikis.