Cross browser add and remove events - with init-time branching. From 'JavaScript Patterns' (O'Reilly) by Stoyan Stefanov.

Cross Browser Events.js

/* Cross browser add and remove events - with init-time branching */

// the interface
var utils = {
	addListener : null,
	removeListener : null
};

// the implementation
if ( window.addEventListener ) {
	utils.addListener = function ( el, type, fn ) {
		el.addEventListener( type, fn, false );
	};
	utils.removeListener = function ( el, type, fn ) {
		el.removeEventListener( type, fn, false );
	};
} else if ( document.attachEvent ) { // IE
	utils.addListener = function ( el, type, fn ) {
		el.attachEvent( 'on' + type, fn );
	};
	utils.removeListener = function ( el, type, fn ) {
		el.detachEvent( 'on' + type, fn );
	};
} else { // older browsers
	utils.addListener = function ( el, type, fn ) {
		el[ 'on' + type ] = fn;
	};
	utils.removeListener = function ( el, type, fn ) {
		el[ 'on' + type ] = null;
	};
}