GreenAddress is a user-friendly multi-signature wallet with improved security and privacy. At no time are your keys server side, even encrypted. For security reasons, you should always use 2FA and the browser extension or Android App.
This wallet requires every transaction to be authorized both by you and this third party. Under normal circumstances, you can regain full control over your bitcoins using your initial backup or pre-signed transactions sent by email.
The developers of this wallet publish the source code for the client. This means any developer in the world can audit the code. However, you still need to trust developers of this wallet when installing or updating the final software because it was not built deterministically like Bitcoin Core.
This wallet can be used from insecure environments. However, this service requires two-factor authentication. This means access to multiple devices or accounts is required to steal your bitcoins.
Prevents spying on your payments
This wallet makes it harder to spy on your balance and payments by rotating addresses. You should still take care to use a new Bitcoin address each time you request payment.
Discloses information to a third party
This service can associate your payments together, log your IP address and know your real identity if you provide personal information like your email, name or banking account.
Tor can be used
This wallet lets you setup and use Tor as a proxy to prevent attackers or Internet service providers from associating your payments with your IP address.
This wallet gives you full control over fees. This means that this wallet allows changing the fees after funds are sent using RBF or CPFP. This wallet also provides fee suggestions based on current network conditions so that your transactions are confirmed in a timely manner without paying more than you have to.