The source code of the current version, which is 5.x, can be obtained from GitHub:
Presentation Engine Git Hub Repo (Current)
Presentation Engine Git Hub Releases (5.x.x)
Snapshots of older and unmaintained versions that didn't live on Github can be obtained via the following links:
Presentation Engine Version 4.1-Deployment (6/23/2014)
Presentation Engine Version 4.1-Development (6/23/2014)
Presentation Engine Version 4.0-Deployment (12/18/2012)
Presentation Engine Version 4.0-Development (12/18/2012)
Presentation Engine Version 3.5-BETA1 (07/07/2008)
Presentation Engine Version 3.1-BETA3 (10/04/2004)
Presentation Engine Version 3.0 (11/12/2003)
Presentation Engine Version 2.1 (12/2/2002)
Presentation Engine Version 2.0 (1/12/2002)
Here is a brief overivew of the evolution of this project. Version 3.0 and below are written as ASP Classic applications. Version 3.1 was my first attempt at moving to an ASP.NET/VB.NET architecture. Version 3.1 was never technically finished although it is functional. Version 3.5 was rewritten and reworked from scratch as a .NET 2.0/C# ASP.NET application with some AJAX thrown in. Version 4.0 is largely a complete rewrite as the application has been transitioned to an ASP.NET MVC4 application. It includes a large amount of client side functionality designed to allow for more streamlined administration of the site. While 12/18/2012 is the original release date for 4.0, it has in fact been updated numerous times since that original release. 4.1 essentially reintroduced the maligned forum feature and fixed a few bugs.
5.0 however is the current version and it marks a complete departure from earlier versions of the project. For starters, it is fully Linux compatible. In fact, I have yet to actually attempt to run it on Windows/IIS. It is written in .NET Core (now renamed to .NET as of version 5.0) and makes use of React.js for all the admin portions of the site. However the actual end user portion is very slim and much effort has been put into making it as quick and as lean as possible.
Legacy versions of the project were distributed without a license. This essentially meant that you could whatever you pleased with it. The current version is being distributed under a GPL3 license however. Why the change? I've gotten old enough that I the need to give something back. Like my code and want to reuse it? Awesome. Open Source your resulting software and we'll all be happy.