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.
Two Factor Authentication
Be safe from malware. Use two-factor authentication by Electrum and Trustedcoin
Split the permission to spend your bitcoins between several wallets
Free and Decentralized
Electrum is free software. Both client and server.
Electrum supports Trezor, the first Bitcoin Hardware wallet
Sign transactions from a computer that is always offline. Broadcast them from a machine that does not have your keys