[jarin]

I prefer 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.

[john-n]

Thats not a very user-friendly site, lots of text, download links are not clear. Keep in mind who your target audience are...