Either something isn't being escaped properly or there's been some new HTML that bodges the site. I'm seeing this behaviour on both the main site and meta.
2 Answers
This issue is appearing for me on Firefox 3.6, Ubuntu 10.04, same as the screenshot posted by the OP. When using chrome, the sidebar appears fine.
From looking at the source, it seems that it's to do with the chat panel on the RHS.
There's some js which tries to load the html for the whole panel, but doesn't seem to escape it correctly, as the browser seems to be interpreting two closing </html>
tags.
$('#ad502-rooms').ready(
function () {
var preloaded =
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTM...
....
</body>
</html>; <-- everything after here getting dumped out in sidebar
displayRooms(preloaded, null, null);
});