There are many user-contributed widgets on CPAN and elsewhere on the Web. Most are registered on the Tk Modules page. Occasionally, this page finds a new home, so the following link, as far as is possible, is guaranteed to point to the proper page:
Other than telling you where these additional widgets are, we won't examine any of them, for various reasons. Mainly, there are too many widgets, some aren't documented at all, and there are questions of support. Use them with these thoughts in mind. Nevertheless, many of those widgets are quality code.