If you use Firefox you can use: https://addons.mozilla.org/en-US/firefox/addon/case-change/
or if you want a quick solution, go to its website: Case Changer
The algorithm is easy only if you’re case-changing English text written in ASCII. For non-English or non-ASCII text, it can be extremely difficult.
Oh, it also occurs to me that rather than finding a plugin for a specific web browser or word processor, you could install a clipboard manager which has a case changing feature. Then you can select text in pretty much any application, hit a key or two, and the clipboard manager will instantly upper/lower/title-case the selection.
Googling for “clipboard manager change case” turned up a few packages which might suit your needs and your operating system. (If you or anyone else is still looking, that is. This thread is already a couple years old.)
Does anyone know how this has been changed? I’m running Windows 7 but I have no “Key settings” choice at that point on the menu. I’d love to disable Caps Lock.
It’s Commodore-Shift, isn’t it? 
I’ve been looking for (well, wishing for) something like this for years. Thank you.
Reported post by harryngh.
Why? He seems to be the author of the plugin but he’s not selling it, and it is pretty much exactly what the OP was asking for.
Yes, the Firefox case changer add in is free and works very well. I’ve used it ever since I started this thread in 2012. Its saved me a lot of grief.