To the best of our knowledge, Impostor is the only web proxy that was designed for use from untrusted network access devices. This, however is not the only reason that makes Impostor unique. To the best of our knowledge, it is also the only web proxy that combines all the following features.

• It performs Single Sign-On and Content Filtering. 
• It is free for non-commercial use.
• It is open-source.  
• It is written in Java - therefore you can run it on any platform that has a Java Virtual Machine (including Windows, Linux, MacOS)
• It is 100% transparent to web sites. This means that websites do not need to change in any way. In fact, they will not even be aware of the fact that Impostor is running.
• It supports many users.  
• It is extensible and pluggable. This means that the software can be easily extended to support
  • more websites
  • any challenge/response mechanism (including token-based authentication such as RSA's SecurID)
  • any credential database component
  • any content filtering component.
• It has been tested with popular web browsers, including Microsoft Explorer, Netscape, Mozilla, Konqueror.

There exist many Single Sign-On systems. According to this taxonomy, Impostor falls into the category of "proxy-based, pseudo-SSO" schemes.