Passlogix v-GO SSO was pretty popular. So popular that it's now being bought over by Oracle. So far, I have participated in 2 projects using Passlogix v-GO SSO. Pretty easy to configure if the user source is Microsoft Active Directory. A lot harder if Sun Directory Server is used as the user source. It's basically a Microsoft-centric product (my personal opinion).
Today, I came across a ESSO product from PasswordBank. Started in 2008, still a pretty young company.
This product differentiates itself by being able to support Desktop Platforms like Windows, Linux and Mac OS. This is something great! ESSO product has always been very Windows driven.