Bitcoin Armory is a full-featured, open-source Bitcoin client promising a dozen features not found in any other client software.
Manage multiple wallets (deterministic and watching-only), print paper backups that work forever, import or sweep private keys, and keep your savings in a computer that never touches the internet while still being able to manage incoming payments and create outgoing payments with the help of a USB key.
Multi-signature transactions are accommodated under the hood and will be completed and integrated into the UI soon.
Armory has no independent networking components built-in. Instead, it relies on the Satoshi client to securely connect to peers, validate blockchain data, and broadcast transactions.