Do "completely general" programming languages exist?

In Ruby, you can add methods to existing classes. Language features like that fascinate me. I want to see what happens when you take this dynamic-ness, or generality, to the extreme. Are there other languages which let you do interesting stuff like that? Something that comes to mind is being able to modify the behavior of all control structures during runtime (I'm not saying this would be useful). What is some other dynamic behavior that would be interesting?

I've had some experience with lisp (scheme) and I think at some point I was asked to write an if function or something (SICP). I don't use scheme much nowadays because it's not as popular.