Xamarin Studio is a new IDE for cross-platform development of applications written in C#. This IDE is designed around the Apple iOS and Google Android SDKs for building, testing, and debugging these applications. Xamarin Studio though is only being offered for Windows and Mac operating systems.
It also appears that this new Xamarin IDE isn't open-source software. Previous to Xamarin Studio they were pushing MonoDevelop as their open-source IDE across all major platforms.
Also announced today was the Xamarin Component Store for "pre-built app components" from UI controls to cloud services. They have also introduced support for developing iOS applications in C# using Visual Studio. Last but not least, they have introduced new pricing tiers for their offering. Xamarin Studio is offered free but for an unlimited application size it's $299 per platform per year. Their business offering is $999 per platform per year and their highest enterprise tier goes for $1899 per platform per year.
More details on the Xamarin announcements can be found via their blog.