Visual Studio 2010 is in the market for a little while. That sounds like no opensource alternative for .Net 4.0 until now. SharpDevelop 4.0 Beta 1 is released. I have tried that. So far, my first impression is slower. I guess that is because the UI is heavy rewritten in WPF. I think most of people will agree WPF got a better appearance and runs slower. But it got a feature I want for long time. Windows forms designer for WPF. Now, you can use the form desginer to edit xaml. That's great! By the way, in release notes, they do not recommend to use the form designer for editing .net 2.0 winfrom files. I tried that, so far, I have not faced any major issues. Well, they don't recommend that, I think you should not do that.

Lastly, so far, I am very happy to the new version. It looks better and got more features!