Electrum is a lightweight Bitcoin client, based on a client-server protocol. It was released on november 5, 2011.
Encrypted wallet: the file that contains your bitcoins is protected with a password. You are protected from thieves.
Deterministic key generation: If you lose your wallet, you can recover it from its seed. You are protected from your own mistakes.
Instant on: the client does not download the blockchain, it requests that information from a server. No delays, always up-to-date.
Transactions are signed locally: Your private keys are not shared with the server. You do not have to trust the server with your money.
Freedom and Privacy: The server does not store user accounts. You are not tied to a particular server, and the server does not need to know you. You can export your private keys.
No scripts: Electrum does not download any script. A compromised server cannot send you arbitrary code and steal your bitcoins.
No single point of failure: The server code is open source, anyone can run a server.
Firewall friendly: The client does not need to open a port, it simply polls the server for updates.
Free software: Gnu GPL v3. Anyone can audit the code.
Written in Python. The code is short, and easy to review.
Support for Bitcoin URIs, signed URIs and Bitcoin aliases