I enjoy sites which provide me with the correct download link for software, based on the fact Firefox places "Linux x86_64" in my User-Agent.

Having said that, the flip side is just as bad, when you get completely rejected from sites because "We haven't tested this site for your browser or operating system". It's a website for crying out loud. I don't get too mad at sites which implement this as long as they provide me a way to continue "at my own risk". However it's the final straw when they flat out refuse to serve me anything other than a page telling me they haven't tested the website for my browser/OS combination... sigh

Edit: This might give some food for thought for AshleysBrain. I like your suggestion, but am curious if we can find a way to send OS and architecture to sites so that they can give me nice download links...

A lot of that functionality can be implemented using JS rather than the user-agent string.

Pretty much just visitor stats (not that those aren't important!)

It doesn't make sense to use User Agent string for anything that someone might have a reason to game since it's so trivial to change them.