Phoenix Online

This easy-to-use plugin allows you to quickly and easily edit post meta values for almost any post type (including custom post types)
Edit post meta as plain text, in textareas, as light-weight HTML or using the full WordPress WYSIWYG editor!
From the first version, this has been designed for ease of use by site Contributors and Editors, but keeps settings reserved for Admin’s only. This gives site Adaministrators control over what less-privileged users can bulk edit.

Bulk Postmeta Editor

This plugin makes it easy to check and edit serialized data in a targeted and comfortable way. Both custom fields and hidden meta data can be easily queried by post type. The plugin then lists all data in an editable input field of your choice which makes comparing and editing the data a breeze.  


Very helpful for debugging

Allows you to bulk-edit post meta information (including custom post types) from a single place.

jphase

WP Modification History enables a metabox that shows modficiation history by date/time, user, and the values that were modified. The modified values are shown in a diff format so that you can easily see what lines and characters were modified, deleted, added, etc.
Within the settings page, you are able to toggle off/on meta values in the event you don’t want certain things to be tracked when hitting the update button on that post type.
Possible future features:
<ul>
<li>Additional settings and display options (AJAX pagination, limit of results per page/metabox, etc.)</li>
<li>Comments and remarks on a particular revision</li>
<li>Email notifications when specific changesets are made</li>
<li>Stats</li>
</ul>

WP Modification History

WordPress Modification History adds a metabox that shows modification history on a particular set of post types.

Bulk Postmeta Editor vs WP Modification History