I've been using the hugely popular Umbraco content management system since v4 and it's worked great every time!
Umbraco is a hugely popular ASP.NET based, open source content management system with great flexibility.
I've been using Umbraco a lot since v4 and I've always been happy with the results, as have my clients.
The number of Umbraco installs is incredible and the developer pool and community is thriving. Even though it's open source it's backed by a profitable company and therefore a great choice of CMS for your project.