[pro] The Best Examples of "Code is Data"

As first, good luck with this list!

I'm in search for best examples of "code is data" paradigm in Common Lisp. For most CL-ers, it probably means "macros", but eval, backquote, anything that processes the code as data is of interest. As "best" I think on the most surprising, powerful, sophisticated examples, not necessarily of a pedagogical value.

Imagine that someone invited you to write the presentation "Five best CL macros ... I seen" or "Five best CL macros ... I wrote." What would you chose and why?

Kazimir Majorinc