I stumbled across this web app from web development agancey Deep Blue Sky a few weeks ago and only had a chance to use it last week but I’m more than impressed.
Get straight to it – http://fmbip.com
Aside from the site’s slick interface and detailed statistics, fmbip let’s you create an account for free and send your client’s a custom URL to visit (such as findmebyip.com/username/your-tag) where this details information is obtained. You can then visit your account after your client has visited the URL and view information regarding their environment such as browser version, OS version, CSS3 support, HTML5 support, HTML5 form fields support and IP information. In fact as soon as you visit the link http://fmbip.com it will output your information for you straight away.
It’s the simplest and most effective way to gather information about selective user’s computers that I’ve come across. Get in to it.