[wonderzombie]

Ruby, especially the standard Array library. Look at stuff like select, each, and whatnot, methods that take a block. Try and write (or rewrite) a toy program you have lying around and make sure you try to use those.

jQuery is another option if you want web-related stuff, esp. http://api.jquery.com/category/traversing/.

Don't thank us for exposing this to you just yet, though. :) You can use this pattern in some mainstream programming languages, but not all of them--- typically it's not idiomatic in Java or C++.