This plugin allows you to place a sort bar, with options to sort the list of posts by any custom field. You can place the sort bar above or below a list of posts in a category or archive or anywhere in you template.
Options are set in the admin panel as to which custom fields to display in the sort bar.
This was originally coded by smartalien on scriptlance. He no longer develops it and I purchased the code from him. It works ok and I put it out there to be used and improved by the wordpress community.