If you run a website that has a paid membership area and unauthorized access is a problem( ie; stolen , abused passwords) then maybe you might want to check out Strongbox...
Strongbox security system features:

* Defense against brute force attacks.
* Defense against password sharing.
* Invulnerable to referer spoofing.
* Compatible with Windows™ Media Player 9 and other video.
* Allows secure links between domains.
* Anti-slurp API.
* Low overhead for high traffic sites.
* Compatible with all CC processors and AVSs.
* No monthly fee.
* reporting and member management module available.
* Professional installation always included.
* Real time open proxy detection option. free bonus for a limited time
* Strong SHA1 password hashing
* Proactive password spider option.
http://bettercgi.com/strongbox/

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