<script type="text/javascript">
	window.onload = function() {
		document.forms[0].username.focus();
	}
</script>

I would also recommend this on any page that has a form element on it that must be filled out. However be careful not to use it on a page that has a form field that the user may not be interested in. A good example of bad placement for this block of code is AVN Ads which sets focus to the search box instead of the username field. Every time I go there I get half way through typing my username and password before it jumps up to the search box and I in my haste I end up typing my password into the search field for all the world to see.