Convert custom fields to tags, categories or taxonomy terms
Custom fields are great for storing various meta data about a post. However, if you routinely need to display posts that have a certain custom field value, you should consider switching to a custom taxonomy, which is better suited for grouping posts.
With this simple plugin, you can convert all custom fields with a certain key to terms in a certain taxonomy, while maintaining the post association.
Note: If you're looking for the old version, see Custom Query Fields