Change Case is a Services menu item which allows you to change the case of any selected editable text in any Services aware application running on your Mac. Most applications are Services aware, such as TextEdit.
Change Case is now at version 1.2. It has two flavours of Sentence case. One for English and one for non-English use. The difference is that in the English language version, lowercase i which appears by itself or immediately before an apostrophe is turned into uppercase I, whereas the non-English version leaves lonely i alone.
Change Case service requires Mac OS X 10.4 (Tiger) or newer.
Here's a screen shot of the Services menu on OS X Tiger:
And on Snow Leopard:
Download Change Case, then install it into either the System Library/Services folder, or your home Library/Services folder. You'll need to log out and log back in to get the new service to be recognized.