Pushup: Encourage your users to keep their browser up to date
This looks like a neat piece of JavaScript that will check the browser your visitor is using, and provide a discrete popup message if there is a newer version available.
It isn't a hard push into installing something other than Internet Explorer, but it will alert them if they're using a browser older than IE7, FireFox 2, Safari 3 or Opera 9.25. This is a beautiful way of encouraging users to use recent versions of their browser, which makes life easier for them (better security) and you (more reliable JavaScript implementation).
http://www.pushuptheweb.com/