There's a growing battle lately on the App Store over "check-in" apps -- Foursquare, Gowalla, and MyTown have all seen big gains in users lately (and a few other contenders have been roaming the periphery), and it looks like we're starting to see some innovation in the mix. Gowalla recently updated their free app [iTunes link], and introduced a new feature called Shared Trips. These are more or less tours (or a line of check-ins at different places around a given city), but they can feature all kinds of things: a set of bars, interesting buildings to see, the old bookstore/coffeeshop run, or any other ideas Gowalla users have. Trips can be shared and rated, and you'll be able to browse through the most popular trips and even earn badges in the app for creating or going on a popular trip.
The new version also adds bookmarks, which will let you remember your favorite check-in spot -- while you can't add them from the mobile app yet, you can browse them from there, and they're hoping to get the functionality in soon.
Obviously, this might not appeal to everybody -- there are lots of iPhone users out there (including me) who aren't so keen yet on sharing their location with everyone they know every time they leave the house. But it is interesting to note that this is a burgeoning mini-industry, brought on almost completely by the ubiquity of the iPhone and its up-and-coming competitors. Any time you have more than a few apps vying for a growing audience, you're going to end up with some interesting innovation. It'll be fun to see what kinds of ideas Gowalla and its competitors can come up with.