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.
I haven't tried mojoPortal, but I love Umbraco.
Things I like:
But really its because I can use it for both small and large projects easily.
mojoPortal seems easier to use to me and it works even with javascript disabled like using noscript browser plugin. Seems more care of accessibility has been taken using progressive enhancement javascript techniques whereas you can't manage your site at all with javascript disabled using Umbraco.
I tried Umbraco and it is not for the timid. I feel I'm a fairly technical person, Sr. Web Developer... and after several hours I gave up.
MojoPortal just works.
It has its flaws, but the simple fact that it just works means it wins.
I actually went for Umbraco in the end and would never look back, its incredibly easy to install and use
To install you can use the web platform installer to install it and the AMAZING amount of free projects you can EASILY install with a couple of clicks make it by far the best CMS out there
http://our.umbraco.org/projects
If you are unsure where to start have a read of this
http://www.blogfodder.co.uk/post/A-Complete-Newbies-Guide-To-Umbraco-CMS.aspx