Electrum is an easy to use Bitcoin client. It protects you from losing coins in a backup mistake or computer failure, because your wallet can be recovered from a secret phrase that you can write on paper or learn by heart. There is no waiting time when you start the client, because it does not download the Bitcoin blockchain.
Instant on: Your client does not download the blockchain, it uses a remote server.
Forgiving: Your wallet can be recovered from a secret seed.
Safe: Your seed or private keys are not sent to the server. Information received from the server is verified using SPV
No downtimes: Several public servers are available, you can switch instantly.
Ubiquitous: You can use the same wallet on different computers, it will auto-synchronize.
Open: You can export your private keys into other Bitcoin clients.
Tested and audited: Electrum is open source and was first released in November 2011.
Electrum has several user interfaces, that share the same wallet code: Classic (Qt), Lite, Gtk, Android, Text (using curses). Go to screenshots to see some examples