Programs for a Sorcerer Supreme

Hello, /prog/.

I know the subject is vague, so let me explain:

I'm confident I know some programming languages very well, but I feel my knowledge is lacking when it comes to many others. So I'm looking for programs that can sharpen up my skills. Stuff that requires one to use most of a language's features. What would you suggest?

C - compilers. Device drivers?
C++ - game engines?
Scheme - an interpreter?
Common Lisp - ?
Haskell - ?
Java - ?
.NET - ?
Scala - ?
Python - ?
Ruby - ?

>>5
A lot of languages go for the small bootstrapper in C and then the full compiler in the bootstrapping version of their language, so it would be 2 sets of compiler bugs tops. If you keep your bootstrapping language clean enough, you'd only need to worry about your own compiler.

And personally, I'll take a high level language over C any day.