EVER TYPED AN ENTIRE EMAIL ONLY TO DISCOVER IT'S IN ALL-CAPS?
Then you need Toggle Case. Toggle Case lets you quickly and easily change the case of selected text to Sentence case, Title Case, Uppercase or Lowercase. Unlike some other extensions, this one works on both HTML Forms and Rich-Text like the Gmail editor, AND retains formatting.
To use this tool, simply select some text then right-click on your selected Text. The 'Toggle Case' menu will appear allow you to change the case to a different style.
Note: there is an issue on sites which use framesets, including iframes. If you find the extension doesn't work then you're probably on a site which is using frames. I'll look at adding support for frames in a new version.