We spent a year designing the new UI and features of Xamarin Studio (previously known as MonoDevelop).
I shared some stories of the process on the Xamarin blog.
After our launch, we open sourced all of the work that we did, as well as our new Gtk+ engine for OSX. Lanedo helps us tremendously making Gtk+ 2.x both solid and amazing on OSX (down to the new Lion scrollbars!). All of their work has either been upstreamed to Gtk+ or in the process of being upstreamed.