According to Wikipedia (we can discuss credibility at some other time) a mashup is a website or application that combines content from more than one source into an integrated experience. There are many kinds of mashups and a short tutorial at Programmable Web presents the basics of creating mashups.
Gill Murrey at a Spiders’ Meeting in 2006 gave a demo of creating a mashup using Google Maps. This seems to be a very common use of mashups and Google Maps. To create a Google Map mashup you need to use their API the details can be found at, http://www.google.com/apis/maps/. From my searching the most complete example of a Google mashup is Stanford University’s campus map. I find this map very complex, hard to read and not very attractive. Another example of a mashup is a list of university podcasts around the world. Again I have a problem that you need to know where what you are looking is before you can find it.
Programmable Web has list of Google map mashups with examples that vary in quality. The ones I tend to like the best use mouse overs on the ballons so you know what each of the little ballons represents without having to click on it.
Given problems such as the maps not being current when you include the satellite view I’m not sure that Google map mashups are ready for prime time. There are definitely cool uses of this technology. Besides Gill is anyone using this on campus?