platform.js seems like a good one file library to do this.
Usage example:
// on IE10 x86 platform preview running in IE7 compatibility mode on Windows 7 64 bit edition
platform.name; // 'IE'
platform.version; // '10.0'
platform.layout; // 'Trident'
platform.os; // 'Windows Server 2008 R2 / 7 x64'
platform.description; // 'IE 10.0 x86 (platform preview; running in IE 7 mode) on Windows Server 2008 R2 / 7 x64'
// or on an iPad
platform.name; // 'Safari'
platform.version; // '5.1'
platform.product; // 'iPad'
platform.manufacturer; // 'Apple'
platform.layout; // 'WebKit'
platform.os; // 'iOS 5.0'
platform.description; // 'Safari 5.1 on Apple iPad (iOS 5.0)'
// or parsing a given UA string
var info = platform.parse('Mozilla/5.0 (Macintosh; Intel Mac OS X 10.7.2; en; rv:2.0) Gecko/20100101 Firefox/4.0 Opera 11.52');
info.name; // 'Opera'
info.version; // '11.52'
info.layout; // 'Presto'
info.os; // 'Mac OS X 10.7.2'
info.description; // 'Opera 11.52 (identifying as Firefox 4.0) on Mac OS X 10.7.2'