The WordPress Categories Widget currently only handles the `category` taxonomy. I’ve modified it to allow the user to select a taxonomy, and tied in the Tag Cloud Widget code to give the user the choice of display formats: list, dropdown, and cloud. Multiple instances are possible, providing the user with an easy method for displaying links to user-built taxonomy terms.
`preg_replace_callback()` is used to fix the JavaScript for the dropdown menu; instead of generating the default `?query_var=value` format, it uses `get_term_link()` to generate the term URL.