Google: 2-Step Verification - Sign in to a Google Account with both a password and a code that the system will send to a landline phone or cellphone, via SMS or call. Backup codes, and offline code generators for Android, iPhone, or BlackBerry also available. Facebook: What Are Login Approvals? How Do I Turn This Setting On? - Instructions on how to require a special login code, sent to a mobile phone, each time a user try to access a Facebook account, from a new computer or mobile phone. Twitter: Using Login Verification - Feature that helps to keep an account more secure. Instead of relying on just a password, login verification introduces a second check to make sure that you and only you can access your Twitter account. Dropbox: How Do I Enable Two-step Verification On My Account? - Security feature that adds an extra layer of protection to a Dropbox account, by requiring a code from a cellphone or from any app that supports the Time-based One-Time Password (TOTP) protocol. Backup codes also available. Apple ID: Two-step Verification for Apple ID - Answers to frequently asked questions about two-step verification for Apple ID. Microsoft: Two-step Verification - Instructions on how to require an extra security code before authenticating to a Microsoft account. PayPal: More Protection for Your Account - Introduces "Security key" and "Mobile phone security key", to protect accounts from unauthorized access by requiring a code from a device that generates codes, or a mobile phone.