Bitcoin is more than just open-source paypal. Its programmable money. Not just int he sense of the code or protocol itself but everything in the eco-system around it. This page is just a brief list of some of the resources currently being developed around bitcoin which allow for interesting concepts to be build using bitcoin and on top of the protocol.
BitcoinJ – Bitcoin Java implementation
bitcoinj is a Java implementation of the Bitcoin protocol, which allows it to maintain a wallet and send/receive transactions without needing a local copy of Bitcoin Core. It comes with full documentation and some example apps showing how to use the library. An example of how to use is is provided here which shows how to write a simple app that forwards any money it receives.