Custom Field Taxonomies is a plugin that lets you use post metadata as ad-hoc taxonomies.

Register taxonomy

For each taxonomy, you have

  • Tag cloud with all values of that field
  • Show only posts with field = value
  • Any theme can add a meta.php template
  • Several template tags (can be found in meta-template.php)

It also has a nice settings page where you can define which fields to be used as taxonomies.

PHP5 is required.