At my day job, we use Sitecore as our enterprise-level Content Management System. We are currently running 9 (or so) sites through it, although we'll have nearly 20 installed by the end of Spring 2010. 

I like Sitecore quite a bit, so when I found out that they offer a free, downloadable personal developer version of Sitecore 6 (the newest release), I just had to get it! Sure, it's a stripped down version…but it still appears to have a TON of functionality, so whatever.

The install itself takes only about 15 minutes and is smooth and easy.  Even with a non-supported version of Vista like I have (Home Professional), the install runs without a hitch.  I even had time to add a custom background image to my Sitecore desktop!

Anyway, if you use Sitecore or are interested in seeing what it's all about, grab the download.  At the very least, your experience installing it will be positive :)

Download Sitecore Xpress