I prefer http://browser-update.org/ http://browser-update.org/ as it is a little less intrusive than a centered popup. Of course, you may want intrusive (like

I prefer http://browser-update.org/ http://browser-update.org/ as it is a little less intrusive than a centered popup. Of course, you may want intrusive (like if your site is horribly broken in IE6) or you may prefer not to direct users to a wordy explanation page, so this is a pretty good option.