This page on drupal.org shows how to use checkboxes and option boxes instead of the drop-down selectors for taxonomy terms when submitting content.

It involves modifying the node edit form using hook_form_alter. According to the page author, it was originally created by merlinofchaos.

I plan on using this technique on some of my other sites, where the users have to suffer with large selection boxes for various taxonomy terms.

Merlinofchaos' AngryDonuts.com post describing an improved version.