Reflexivity and Introspection
A fascinating niche of programming languages consists of those languages which are constructed in themselves. For instance, Squeak is a Smalltalk whose interpreter is written in Squeak. Likewise, the best language for writing a LISP interpreter turns out to be LISP itself. (That one is more like nesting than bootstrapping, but it's closely related.)
I think Ruby has enough introspection to be built the same way. Recently, a friend clued me in to PyPy, a Python interpreter written in Python.
I'm sure there are many others. In fact the venerable GCC is written in its own flavor of C. Compiling GCC from scratch requires a bootstrapping phase, by compiling a small version of GCC, written in a more portable form of C, with some other C compiler. Then, the phase I micro-GCC compiles the whole GCC for the target platform.
Reflexivity arises when the language has sufficient introspective capabilities to describe itself. I cannot help but be reminded of Godel, Escher, Bach and the difficulties that reflexivity cause. Godel's Theorem doesn't really kick in until a formal system is complex enough to describe itself. At that point, Godel's Theorem proves that there will be true statements, expressed in the language of the formal system, that cannot be proven true. These are inevitably statements about themselves---the symbolic logic form of, "This sentence is false."
Long-time LISP programmers create works with such economy of expression that we can only use artistic metaphors to describe them. Minimalist. Elegant. Spare. Rococo.
Forth was my first introduction to self-creating languages. FORTH starts with a tiny kernel (small enough that it fit into a 3KB cartridge for my VIC-20) that gets extended one "word" at a time. Each word adds to the vocabulary, essentially customizing the language to solve a particular problem. It's really true that in FORTH, you don't write programs to solve problems. Instead, you invent a language in which solving the problem is trivial, then you spend your time implementing that language.
Another common aspect of these self-describing languages seems to be that they never become widely popular. I've heard several theories that attempted to explain this. One says that individual LISP programmers are so productive that they never need large teams. Hence, cross-pollination is limited and it is hard to demonstrate enough commercial demand to seem convincing. Put another way: if your started with equal populations of Java and LISP programmers, demand for Java programmers would quickly outstrip demand for LISP programmers... not because it's a superior language, but just because you need more Java programmers for any given task. This demand becomes self-reinforcing, as commercial programmers go where the demand is, and companies demand what they see is available.
I also think there's a particular mindset that admires and relates to the dynamic of the self-creating language. I suspect that programmers possessing that mindset are also the ones who get excited by metaprogramming.