DotNetNuke is an open source, ASP.NET web portal application that I've spent a lot of time with since March 2003 - only 3 months after the project's initial release. One of the things I do is to write and teach DNN as an Application Development Platform, and I've put together quite a bit of material on DNN over the years. My MSDN Webcasts and developer group DNN presentations are here, which you can download for your reference.
DotNetNuke 5 has finally released to the public. You can download it here: http://www.dotnetnuke.com/tabid/125/default.aspx (site registration is required). DNN v5.0 has many new features, but most aren't yet exposed to the DNN portal user or admin - much of the v5 work to date has been in building new foundations for future changes.
A team of Florida, USA developers, including myself, put in many hours during the last half of 2008 writing new edition of the WROX Professional DNN 5 book - now in its third edition. Check the book out for technical details of v5 changes, portal administration, module development, and many other things. You can find the book on Amazon or Barnes and Noble.
Although it's not meant to be a major DNN resource, I soon hope to get some time to update the links and information on this page to be more current. Please bear with me.