There's also libbitcoin, which aims to rewrite bitcoin, make it super-pluggable, very easy to do and hack everything at every level, and very configurable:
http://forum.bitcoin.org/index.php?topic=30646.0
https://gitorious.org/libbitcoin/libbitcoin

And don't forget justmoon's javascript bitcoin which is based off of BitcoinJ:
https://github.com/justmoon/node-bitcoin-p2p/