CalLite is an interesting example of a control that was developed entirely by contributors to this forum, attempting to meet a need originally expressed by a hobby programmer living in Israel who could not find a Pascal calendar component that could cope with Hebrew (right to left script). It is easy to trace the history of its development here.
wp, who maintains CalLite, has managed to weave together the best ideas from several contributors, and also documented the result.
Of course, as with all open source code, the ultimate documentation is the source itself.