<script type="text/javascript" src="http://dren.ch/js/strftime.js"></script>
<script type="text/javascript">
 var d = new Date();
 alert(d.strftime('Today is %A %B %e, day %j of the year %Y'));
</script>

Please refer to the FreeBSD strftime documentation if you are not familiar with the format codes.

"Known issues":

• I made no attempts at i18n or l10n
• Z and z worked under spidermonkey in a kludgy way and broke under IE and Safari, so they're commented out for now
• Still missing %U, %V, and %W, and I also made no attempt to implement %E* and %O*
• As a bonus of sorts, Number objects get a new pad() method (read the code if you're interested)
• Overall it's only "lightly tested"
• Somebody already did this

January 25, 2008 update: I never specified a license for http://dren.ch/js/strftime.js; it is public domain.