Electrum is one of the very first bitcoin-wallets. Since its launch, it has released programs for different OSes (in the form of a "thin" wallet) and smartphone apps. Electrum wallet has an open source code and is compatible with Tor browser -- this is great for users who are interested in a high degree of anonymity. According to Electrum's developers, about 10% of all Bitcoin transactions are sent using the wallet.
A strong password and a seed-key are required with your initial sign-up. Private keys are stored by the users, regardless of whether the desktop or mobile wallet is used. Technical support team actively engaged with users.