CyberSheath- How Multi-Factor Authentication Protects Your Business

SANS Whitepaper – Two-Factor Authentication (2FA) using OpenOTP

Duo Multi-Factor Authentication

RSA SecurID Access

Thales

Symantec Whitepaper – Two-Factor Authentication

Duo – Two-Factor Authentication Evaluation Guide

Schneier on Security – More on Two-Factor Authentication

BrightTALK – Two-Factor, Too Furious: Subverting (and Protecting) Multi-factor Authentication

SANS Whitepaper – Two-Factor Authentication: Can You Choose the Right One?