One of the most underrated Google mobile Web apps is Google Maps. Most smartphones and tablets have a native app for Google Maps, so a mobile Web app doesn't seem necessary. Unfortunately, native apps aren't always updated frequently and there are many missing features. Google does a good job at updating Google Maps for Android, but Apple's Maps app for iOS rarely includes new Google Maps features.
That's probably one of the reasons why the Google Maps mobile site was updated to include most of the features from the desktop site. Another reason is that Google wants to offer "a consistent Google Maps experience wherever you use it."
The updated Google Maps mobile site has features like local business search, Google Places, driving directions, layers, My Maps, starred locations, search suggestions. If you can't find biking directions or information about businesses in the Maps app for the iPhone, you can go to maps.google.com in your mobile browser and use these features.
Just like the mobile YouTube site, "Google Maps for mobile browsers is platform independent - you will always get a consistent experience and the latest features without needing to install any updates, no matter what phone you use."
I've tried the updated mobile interface on an iPhone 3GS, a Nexus One and an iPad 2. While all the new features are great, the site is still too slow and unresponsive to be useful. Until Google solves performance issues and mobile browsers become more powerful, people will still use the native app.