You want to see what a good mashup really looks like? Check out Atlas, from Fresh Logic Studios. It's a mapping tool that uses the Microsoft engine behind Windows Live Local, but the Atlas team has layered in data from elsewhere: traffic, from Yahoo; blogs, from Feedmap.net; events, from Eventful; Wi-Fi hot spots, from HotSpotHaven; and more.
The beauty of Atlas is that the data from external sources is all presented in a consistent and beautiful user interface. It's not perfect (two gripes: first, the transition between map and bird's-eye view is awkward; and second, why do I have to put in a zip code each time I want to look for local data, when clearly what I want is data from the map area that's already displayed?), but the core of the design is solid and flexible. I would not be surprised to see the interface refined and the number of data sources expanded; the UI could handle it.
With its slick user experience, Atlas competes with the new Yahoo Maps beta, but Atlas offers more data. Both of these products (Yahoo and Atlas) are getting curiously close to the downloadable app [download] in power and capability. All the online maps really need right now is a 3D engine. I don't think it will be long before we see that. The Flash-based shooter game Phosphor [blog post] shows us that you can do almost anything--even display moving 3D graphics--in a Web browser.