Travel guides are good for getting an overview of a place that you're visiting - but they don't tell the whole story. That's why geotagging is becoming increasingly possible, as it allows people to share interesting places that you probably won't read about in a book.
Geocast allows you to share files with other users based on location and proximity from your Android phone. You can view text, images and URLs, which are tagged to a precise location on a map. All of the data that is uploaded is checked by the Geocast team, so you can be sure that it's been verified independently.
The goal of Geocast is to create a community of users that share geopositioning data. Once the community really takes off it will be great for finding interesting sights, restaurants, or things to do in an area that you're visiting.
You should note that Geocast is still very much a 'work in progress'. At present you can view content for a few different locations and upload your own basic shareables, such as text, photos, URLs and YouTube links, but it's still not possible to upload your own audio or video, and the uploading system is pretty basic.
Once more functionality is added to Geocast though, it looks like it could become an essential travel companion.