My favorite example would have to be CLOS the Common Lisp object system. Although it's a bit hairy in certain places, it essentially transforms a procedural/fun

My favorite example would have to be CLOS the Common Lisp object system. Although it's a bit hairy in certain places, it essentially transforms a procedural/functional language into on of the most powerful object oriented languages around. Macros play a small, but crucial part of it, subverting the rules of lisp in order to add another dimension to the language. Another great example might be Postmodern and CLSQL, both of which include SQL DSLs, much better than concatinating strings or muddling around with ORMs. CL's loop macro basically ads an algol like language to lisp(if you dislike loop, there exist lispier alternatives like iterate). Macros take care of the case between writing a function and writing an interpreter, and with judicious use, they can help build much cleaner abstractions that give a qualitative improvement to a code base.