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