The WiKID Strong Authentication System is a highly scalable, secure, self-hosted two-factor authentication system. It is simple to implement and maintain, allows users to be validated automatically, requires no hardware tokens, has a simple API for application support (via Ruby, PHP, Java, COM, Python, etc.), supports multiple domains, and supports replication for fault tolerance and scalability. It also supports mutual /host and transaction authentication, wireless tokens only domains, locked tokens (to your PC), anti-keystroke logger keypad PIN entry, etc.
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Latest two-factor tutorial:
Add two-factor authentication to Google Apps using WiKID.