If you’ve ever created many taxonomies for a post type, you know that the add/edit screen can get very unwieldy with all of those metaboxes, especially with hierarchical taxonomies. And then sometimes you have something that is a taxonomy but doesn’t need items added very often, especially in client work where there are often predetermined options that may occasionally be added to by the client, which can be done in the taxonomy’s admin screen.
This is where we can use Chosen, a JS plugin to make multi-selects much more user-friendly, and just so happens to blend right in to the WordPress admin. You can take multiple taxonomies and put them into a single metabox. It’s not limited to taxonomies, either – you can use it for post meta (custom fields) as well. I’ve used it to select taxonomy terms for which a post is “essential” – optgroups came in very handy there. I actually would love to see tag selection look and behave more like this in core, but there would be more work to be done for adding new terms.
The basic idea is that you enqueue the script and style on the appropriate admin page (NOT all admin pages) and then initialize the field(s) based on a class or ID. I usually just put the initialize right in the metabox itself, but you could always be clean and put it in the admin_head if you want. From there, you show all terms in the taxonomy in a select field (including empties) and use the nifty selected() function to show which ones are associated with the current post. When saving, it’s as easy as wp_set_post_terms() after the usual checks for nonce, etc. You’ll also want to hide the original taxonomy metabox(es) as well, of course.
Full Github Gist (updated July 10, 2012):