What's your favorite macro?

[Benjamin.Pierce@PROOF.ERGO.CS.CMU.EDU]

I'm interested in examples or applications that clearly demonstrate
the utility of macros as a language extension mechanism.  

In particular, I'd like to know about useful macros with the following
properties: 
   1) They make "significant" use of the macro definition mechanism (this
      is to rule out kinds of notational extensions that could be handled
      just as well by a simpler mechanism).
   2) They cannot be implemented as functions (this rules out uses of
      macros to improve efficiency of programs without increasing the
      expressive power of the language).
   3) They provide features or constructs that would NOT be a desirable
      addition to the base language (this rules out macros that "should"
      be language features rather than macros, e.g. delayed evaluation, 
      polymorphic types, records, exceptions, ... [I know some of these 
      are debatable!]).

Pointers to previous discussions or research papers also gratefully
accepted.

	Benli