Available for Redhat Linux, Secure Server was originally sold as a standalone product. Now with Release 6.1, the Secure Server 3.1 is included with the Professional Version. Version 2.0 was easy to install and setup, but the documentation was somewhat spartan. I have worked with Enterprise on Solaris. The Java administration tool makes managing this beast very easy.
Documentation is outstanding.
Pricing is concurrent connection based. Despite rumors to the contrary, I also use Microsoft products. Internet Information Server is available with NT Server. It is easy to set up and maintain. Remote administration can be a hassle.