From a simple User-Agent, it tells you what might be the browser, version and platform used.
Uses whatismybrowser.com
service to retrieve those information.
$ python whatisthisbrowser.py "Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:35.0) Gecko/20100101 Firefox/35.0"
Looking for User-Agent: Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:35.0) Gecko/20100101 Firefox/35.0
Firefox 35 on Ubuntu Linux