Run CSharp is a port of the Mono 2.10 compiler to Silverlight 4.
Try it out at: http://runcsharp.com That site features the compiler plus a list of "run blocks". Just type an expression or class declaration into the box, hit Enter, and watch the compiler first compile the code then execute it.
The port is the product of only 6 hours of work, so please be weary of bugs.
-Frank A. Krueger (@praeclarum on Twitter)