Mass Custom Fields Manager is a plugin by Oren Yomtov to help people who use WordPress to manage their post’s custom fields with ease.
If you don’t want to spend your time editing each and every single post’s custom fields to the same value or you want to delete/update specific posts you don’t have another tool that let’s you do that.
You can choose posts by category,ID,custom field value,tags (since v1.0) and more and with a click of a button manage their custom fields.
You can also choose tags that if a post is saved having that tag, it will automatically get a specific custom field with a specific value (Added in v1.0).
Download and info: http://wordpress.org/extend/plugins/mass-custom-fields-manager/