WebKnight is a very popular and open source WAF for IIS. WebKnight blocks known exploits and 0-days by detecting HTTP protocol violations and by limiting parameters sent to your web application. Scanning for the OWASP Top 10 attack signatures and a lot more that we've seen since the year 2002 when we started this GNU GPL project.

ThreatSentry is a Web Application Firewall and Intrusion Prevention solution that helps system administrators improve web application security and comply with regulatory demands such as Section 6.6 of the Payment Card Industry Data Security Standard. ThreatSentry 4 supports Windows Server 2008 R2 and IIS 7 on 32 and 64 bit systems. An ISAPI Extension hosted in MMC, ThreatSentry’s knowledgebase of pre-configured filters is designed to identify and block a broad range of web application threats...

With AuthXt, you can extend your web server's functionality with a custom authentication mechanism. You can password protect any type of resources on your site (directories, videos, images) authenticating users against different type of back ends.