bugfix: if handler do waituntilexists() again with same selector then new IntervalId will replace old one in intervals array before removeListener() call. So new Interval will be cleared instead of old, and old Interval will never be cleared.

jquery.waituntilexists.js

;(function ($, window) {

var intervals = {};
var removeListener = function(selector) {

	if (intervals[selector]) {
		
		window.clearInterval(intervals[selector]);
		intervals[selector] = null;
	}
};
var found = 'waitUntilExists.found';

/**
 * @function
 * @property {object} jQuery plugin which runs handler function once specified
 *           element is inserted into the DOM
 * @param {function|string} handler 
 *            A function to execute at the time when the element is inserted or 
 *            string "remove" to remove the listener from the given selector
 * @param {bool} shouldRunHandlerOnce 
 *            Optional: if true, handler is unbound after its first invocation
 * @example jQuery(selector).waitUntilExists(function);
 */
 
$.fn.waitUntilExists = function(handler, shouldRunHandlerOnce, isChild) {

	var selector = this.selector;
	var $this = $(selector);
	var $elements = $this.not(function() { return $(this).data(found); });
	
	if (handler === 'remove') {
		
		// Hijack and remove interval immediately if the code requests
		removeListener(selector);
	}
	else {

		if (shouldRunHandlerOnce && $this.length) {
			// Element was found, implying the handler already ran for all 
			// matched elements
			removeListener(selector);
			
			// Run the handler on all found elements and mark as found
			$elements.each(handler).data(found, true);
		}
		else if (!isChild) {
			// Run the handler on all found elements and mark as found
			$elements.each(handler).data(found, true);
					
			// If this is a recurring search or if the target has not yet been 
			// found, create an interval to continue searching for the target
			intervals[selector] = window.setInterval(function () {
				
				$this.waitUntilExists(handler, shouldRunHandlerOnce, true);
			}, 500);
		}
	}
	
	return $this;
};
 
}(jQuery, window));

jquery.waituntilexists.min.js

(function(e,f){var b={},g=function(a){b[a]&&(f.clearInterval(b[a]),b[a]=null)};e.fn.waitUntilExists=function(a,h,j){var c=this.selector,d=e(c),k=d.not(function(){return e(this).data("waitUntilExists.found")});"remove"===a?g(c):(k.each(a).data("waitUntilExists.found",!0),h&&d.length?g(c):j||(b[c]=f.setInterval(function(){d.waitUntilExists(a,h,!0)},500)));return d}})(jQuery,window);

Evgeniy: Could you please clarify what the license for this code is or whether it's in the public domain?