Since you are here searching for useragent (UA) strings, I take the privilege to consider that you already know about user agents, what they do and how to change them. For those who don't, I would like to give a very short layman definition for same. Useragent is nothing but a string in your browser containing details about your operating system, browser type and browser version.
Eg of a user agent string:
Windows-RSS-Platform/2.0 (MSIE 9.0; Windows NT 6.1)
Here MSIE suggests the Browser, 9.0 is the browser version and Windows NT 6.1 is the Operating system
There may be other details to it but I'm sure you would not need those unless you are a very advanced user.
Coming back to useragent strings. Here are some few examples why you would want to access useragents and change them.
Lets say you come across any of the following situations:
1) You are a Mac user, but wish to visit a website which looks good on a windows operating system (OS).
2) You wish to check a mobile website or download a mobile content on your PC
3) You are a webmaster/developer and wish to test your web content on multiple browsers and OS.
4) You wish to view some webcontent which is not targeted to your OS
This list can go on and may vary from beginer to advanced users.
As a layman if you are faced with any of these situations your first mode of action would be to look for a different system that can solve your problem, but if you are aware of user agents, you'd probably want to spoof your web browser to behave as you want it to be.
Just add the UA string to your browser and you are ready to go.
Here is a list of useragents that you would like to try your hands on.