With new version of WP this summer (3.6 oscar), now new version of xili-tidy-tags plugin:

As example, the source now contains a new template tag – xili_tidy_tags_dropdown – to display a tag cloud as a dropdown list as expected by some users.
In my opinion, dropdown html tag is a little strange way and a flowed solution combining html and a little javascript (requiring jQuery) to redirect to the link associated with the tag (like in a cloud).

The example code is at (around) line 1009 in file of the plugin. The syntax is very easy like another plugin tag for cloud with a param (id_dropdown) to set the unique ID to a unique value for jQuery.

Soon more explanations…

M.