The Flag module has one omission: when an item is flagged by several users (assuming it's a non-global flag), there's no easy way to clear all these flaggings at once. This module provides a "Reset link" field for Views for doing this.
Note: This module does not display this link under each node, but
you can install the Custom Links module to achieve this: set your
link to flag/reset/bookmarks/[nid] (replace bookmarks with the name
of your flag).
This module also provides a "Remove flagging" link, for Views, for clearing a specific flagging.
You must use Flag 2.x. This module is not compatible with Flag 1.x.
You can download this module here.
http://drupal.org/project/flag_clear
After enabling this module you'll find two new pseudo fields in your Views admin interface:
Flag: Reset link
This removes all the flaggings for this item. This field sits on the "Flag counter" relationship (because it wants to know whether anybody has flagged this item), so you'll have to bring in this counter relationship first.
Flag: Remove link
This removes a single flagging (The "relationship" record).
Users have to have the "administer flags" permission to be able to use these links.
(And see the note above, about Custom Links, for displaying a reset link under each node.)