Hamed Momeni

this plug-in displays the Browser and Platform of user who commented in your blog. it is capable of determining the version of the borwser.

User Agent Displayer

the User Agent Displayer plugin displays the Browser and the operating system, user used to leave a comment.

trauben-nuss

This plugin adds a abbreviation of the operating system (win, mac, linux, iPad, iPhone) to the body-tag. 
So if there’s a OS-specific difference, you can easily react to it by using the css-selector (e.g. “.mac”).
Next steps of this plugin: 
* adding a settings page 
* add the class to specified elements 
* add user-language-abbreviation to specified elements
let me know, if you wish something helpful.

OS Adder

Add the operating-system's abbreviation as a CSS-Class to the body-tag.

User Agent Displayer vs OS Adder