I have been look around for Free/Open Source ASP.NET CMS / Portal systems for a while now, and have seived it down to two different ones.
Umbraco - http://umbraco.org
I used Kentico, DNN, Sitecore, Joomla, CMS Made Simple (Yes admittedly not mojoPortal). Umbraco is by far the most powerful if you are after a highly customised and highly specified solution. Linq2Umbraco just seals the deal.
However, if you are after idiot proof CMS with everything built in, and your biggest concern is to look for check boxes to enable forum/blogs/whatever other joke modules/bells and whistles/etc. Umbraco isn't for you. IMO Kentico/DNN are the ones.
Edit - And 3 years later, I've used SharePoint, epiServer, SiteFinity as well.
Umbraco still wins hands down.