As simple as this:
if($.browser.msie && $.browser.version=="6.0") alert("Im the annoying IE6");
Update
Please note that $.browser is removed from jQuery 1.9
If you still need to use $.browser in jQuery 1.9 (or other deprecated functions), try jQuery-migrate (https://github.com/jquery/jquery-migrate/ – http://code.jquery.com/jquery-migrate-1.2.1.js)