If you forgot how much you owe from that last group dinner, then you need MoochSpot. It helps you and your friends easily track expenses. Built on the Facebook API.
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Google APIlity Library
C# Wrapper for Wesabe
Facebooker: Ruby library for Facebook
Official Facebook .NET Client Library
Minifb: Python library for Facebook
APIs, mashups and code. Because the world's your programmable oyster.
Executive Editor, ProgrammableWeb. Author, Map Scripting 101. Lover, APIs.