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 - ?

>>11
I wouldn't call Common Lisp as purely functional. It's designed to support functional programming, but you can do imperative programming in it just fine. CL is also very nice when it comes to metaprogramming, and it can be used for low-level tasks too with implementation-dependent extensions(FFI...). It's also nice for writing compilers and interpreters as runtime code generation is defined in the CL standard, thus you can compile to CL and just leverage the native code compiler rather nicely (of course, you can write full fledged compilers just as well, but you can do that in any real programming language...).

tl;dr: Common Lisp is a mudball where all kinds of programming paradigms are mixed in, and you're free to use whatever best fits your style.