Electrum

Electrum is a lightweight Bitcoin client, based on a client-server protocol. It was released on november 5, 2011.

Encrypted wallet
Deterministic key generation. If you lose your wallet, you can recover it from its passphrase.
No blockchain download; the client requests that information from a server
Private keys are not shared with the server.
Firewall friendly: The client does not need to open a port, it simply polls the server for updates.
No single point of failure: The server code is open source, anyone can run a server.
Python. Very compact code, easy to contribute.

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