Haggle

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Haggle is a networking system that allows mobile devices to exchange content directly between themselves whenever they happen to come in close range. The content to exchange is based on the interests that users declare. Haggle supports both Bluetooth and WiFi connectivity (Bluetooth currently requires root).

Haggle runs as a background service and applications have to be specifically written to take advantage of the communication it offers. PhotoShare (found on separate market page) is one such application that exchanges pictures between mobile phones based on users' interests.