Kneedle is a web API based application for buying, selling and communicating locally. You could compare it to Craigslist.
Some of the unique features are a map based view, the ability to search by distance, and a saved search option which will alert you when someone posts an item nearby that you have previously searched for.
The kneedle.js file is the node.js API, it uses MongoDB
The kneedle-titanium-ios-android-app folder contains the source for the Titanium app which can compile to iOS or Android.