ToDo is often used in a PersonalInformationManager for things which have not been completed. This is a more universal version than that of comments made in source code. It is often scratched down on the back of envelopes, napkins from lunch, a 3 by 5 card, or in a formal notebook dedicated to Time and Information management.
Enjoys a small amount of use to tag ToDo items on some wiki pages; click on the title of the page and maybe you can help the author out.
TODO is often found in code indicating that something isn't completed yet. This has become so common that there's now an official @todo JavaDoc tag. Most pages on the wiki citing ToDo are using the term in this way.