Favorite Programming Language Features?

johnnyb asks: "I'm curious what everyone's favorite programming language features are. I'm looking for both the general and the specific. I'm especially looking for features that few people know about or use, but are really useful for those who do know about them. What are your favorite programming language features?" "A couple of examples to kick off the conversation:

• Continuations
  
  Continuations are very interesting, because they can be used to implement a number of flow-control features such as exceptions, coroutines, cooperative multithreading, and are better at modelling web interactions. This is a more general feature, but most people use these in conjunction with either scheme or ML.
  
  
• Tuple-returning
  
  It is a huuuuge time-saver when languages like Perl allow functions to return tuples. Instructions like '($a, $b, $c) = $sth->fetchrow_array()' is a wonderful thing.
  
  
• The flip-flop operator [Perl's '..' operator]
  
  Another perlism that I just think is cool. Read more about it here.
  
  

Okay, on to yours!"