Question: One of the requirements of one of my clients is to show a message to a user that's using an older version of Firefox, Chrome, IE or Safari. For the last two it's easy (IE 9+ and Safari 5+). But for Firefox and Chrome, the requirement is that the site supports the current and the previous version of each browser.
To get the clients browser and version is quite simple, but I need to know what is the last version of each browser. Is there any way of doing that?

Answer: 
Well, technically, all you need is a place that keeps track of what the latest version of each browser (Ideally with a web service).

A quick Googling I was able to find: <http://www.webdevelopersnotes.com/articles/latest-browser-versions.php>

It's not a web service, but you could simply parse that HTML and get the latest version of each browser. Alternatively, you could keep track yourself and manually update this information in your database.
You should only ever have to worry about IE when it comes to version enforcement. Exception being, of course, if you need a specific API that's just not available in previous versions of Chrome/Firefox, but in that case you'd look and enforce the API, not the browser version. Adding to that, if Chrome/Firefox don't have an API, chances are even the most bleeding edge version of IE won't either, so the point is moot. The fact is Firefox and Chrome, and in part Safari, are far ahead of IE when it comes to browser hosted capabilities that trying to enforce specific versions of the former would likely be a lot of wasted time and effort to yield nothing substantial.