C# 4.0 with a REPL (Read Eval Print Loop)
C# 4.0 will apparently have significantly more powerful dynamic programming language features.
I found the slides talking about the "C# compiler as a service" extremely intriguing so I watched the video for more information. Well the demo he gave at 61 minutes was really fascinating. He actually wrote a read-eval-print loop for C#, by using the C# compiler as an API.
There are some interesting parallels with Heron, but the fundamental difference is that Heron provides the code object model at compile-time. This has the potential for creating extremely efficient code, which is optimized statically: not just limited to the minimal optimizations provided by the JIT.