Recently elav explained the how to change the User Agent in Firefox, here I will explain how to do it with Opera and that this also show the distro we have.
It happens that in my case I use ArchLinux y Opera as the main browser, when I leave a comment here on <° Linux for example, before I made this change, it came out as the browser I use Opera, yes, well up there, but as a distro or GNU / Linux Operating System only, when should I get out Opera y ArchLinux.
So they can see the difference EAST is a comment with Opera without having made the change that I will show you here, while THIS OTHER Yes, I was already counting on the small change 
To configure Opera and show it as a distro ArchLinux, here the steps:
1. We open Opera
2. In the address bar we put - » about: config
3. It will open a menu of options, in the search bar we write «id»(Without quotes), the following will be shown:
4. We go down a bit until we find this:
5. And there we put the following:
Opera-Next/12.00-1116 (X11; Arch Linux x86_64; U; en-us) WebKit/532+
6. Then we must close and reopen Opera, the change will be done 
This is to have our User Agent completely customized, just how we want it, however Opera offers us several predefined options. That is, if we simply want to configure Opera to make it look like we're sailing with Firefox, we won't have to put such a long or "complex" line.
Here are the steps to configure Opera as Firefox or Internet Explorer 
1. We open Opera
2. In the address bar we put - » about: config
3. It will open a menu of options, in the search bar we write «agent»(Without quotes), the following will be shown:
4. As you can see, I pointed out a number that by default is 1, if they change that #1 for #2 then its User Agent will say that they sail with Mozilla Firefox, if they change it for a #3 would Internet Explorer.
And well, that's all 
Any doubt or complaint, question, idea, suggestion… let me know 
regards