JavaScript: practical partial application, like creating a function that can be passed to Array#filter or the like.

filterify.js

/*
 * Function#filterify
 *
 * Copyright (c) 2012 "Cowboy" Ben Alman
 * Licensed under the MIT license.
 * http://benalman.com/about/license/
 */

Function.prototype.filterify = (function() {
  // IIFE gives us a place for storing local vars...
  var arraySlice = Array.prototype.slice;
  // ...and keeps the local "filterify" NFE name from leaking in IEs.
  return function filterify() {
    var fn = this;
    // "Pre" arguments.
    var pre = arraySlice.call(arguments);
    // "Post" arguments.
    var post = [];
    // Search the arguments for placeholder.
    var idx = pre.indexOf(filterify);
    // If placeholder was found, adjust both pre and post so that they
    // are the arguments before and after the placeholder.
    if (idx !== -1) {
      post = pre.splice(idx, pre.length - idx).slice(1);
    }
    // The returned function will be passed to Array#filter or the like and
    // should only accept one argument.
    return function filterified(value) {
      // Invoke the original function, inserting the passed-in value wherever
      // the "filterify" placeholder was (or at the end).
      return fn.apply(this, pre.concat(value, post));
    };
  };
}());

usage.js

var foo = function(a, b, c) {
  console.log(a, b, c);
};

foo("a", "b", "c");
// a b c

console.log("---");

var __ = Function.filterify; // Placeholder reference

["_", "__", "___"].forEach(foo.filterify(__, "b", "c"));
// _ b c
// __ b c
// ___ b c

["_", "__", "___"].forEach(foo.filterify("a", __, "c"));
// a _ c
// a __ c
// a ___ c

["_", "__", "___"].forEach(foo.filterify("a", "b", __));
// a b _
// a b __
// a b ___

console.log("---");

["_", "__", "___"].forEach(foo.filterify());
// _ undefined undefined
// __ undefined undefined
// ___ undefined undefined

["_", "__", "___"].forEach(foo.filterify("a"));
// a _ undefined
// a __ undefined
// a ___ undefined

["_", "__", "___"].forEach(foo.filterify("a", "b"));
// a b _
// a b __
// a b ___

Good call. It's hard to keep track of method signatures when MDN also lists SpiderMonkey extensions.