BitCoinJ implements the native BitCoin P2P protocol, which allows it to maintain a wallet and send/receive transactions without needing a local copy of the official implementation. It comes with full documentation and some example apps showing how to use the library.
The project aims to be easier to understand than the C++ implementation, and be suitable for usage on constrained devices such as mobile phones.
BitCoinJ implements (or rather, will implement) the "simplified payment verification" mode of Satoshis paper. It does not store a full copy of the block chain, rather, it stores what it needs in order to verify transactions with the aid of an untrusted peer node.
Выглядит достаточно завлекательно.