Script Summary: Allows you to unfavourite deviations while editing your Favourites.
When editing your Favourites, the script adds a "Remove from Favourites" option to each thumbnail's blue menu. This option completely removes the deviation from your Favourites, and it allows you to clear out large numbers of deviations from your Favourites without having to go to each one's page.
Furthermore, once you've unfavourited something, a little box will appear in the upper-right corner of the page listing what you've unfaved, so if you accidentally unfav something, just use the list to find it and re-fav it. (Leaving Edit Mode hides the list, and leaving or refreshing the page clears the list.)
Important Update 4/26/2009
Fixed a bug that rendered the script completely unusable; also improved the script's code, and added the Unfav'd Deviations list (described in the second paragraph here).