Browser extension | Metafield sets | Bulk editor,import/export
Popular with businesses in the United States
Seamless workflow: use directly in Shopify admin
Speed tested: no impact to your online store
The most neat free metafields editor I've seen so far. Very unfortunately the current version doesn't seem to be useful as it doesn't provide bulk editing feature (unless you've got just a few products in your store, but in this case you can probably survive without metafields at all).