It's the "Annotate" page linked off a gallery management page in either Scrapbook (available if you have a paid account) or PicPix (available if I gave you an account).
-- very quick/usable.
-- film strip animates when you click an icon, and centers the item in the visible area
-- tags smartly auto-complete. you can hit "," to accept a completion and move on to the next tag.
-- pressing enter saves the current tags/title/description, advances to next picture in given filter, and sets focus back on tags
-- tags data model adapts as you use it, reweighting priorities
-- can filter on All, Untagged, or Untitled
-- mouse wheel works on Firefox in the left filmstrip div, where it normally wouldn't, being a div.
-- no page heading... just breadcrumbs. don't waste your screenspace.
-- no scrollbars on page. utilizes your screenspace as best it can.
-- auto-chooses 320 or 640 version of enlarged preview for right side, depending on how big your browser's screen is. will chop off some from a 640, but at a certain points, kicks down to 320.
-- left hand filmstrip thumbnails lazily load when they come into view, so the page loads fast, and you can manipulate huge galleries without downloading a ton of thumbnails at once.
-- thumbnails lazily load even lazier when actively scrolling (in IE and Firefox), waiting 100ms for you to hold position before starting to load. (so scrolling around rapidly from top to bottom doesn't kick off 10,000 loads)
-- saves enqueue in background. you can see processing with a spinning hourglass next to each item. on any server error, saving is retried.
-- clicking big preview on right brings up pic management page in new window (or if you middle click, in new tab, if browser so configured)
-- works in multiple site templates (sounds easier than it was)
And that's just the obvious stuff I can think of. Tons of little details went into this, as it's the primary page I've wanted forever to annotate (tag/title/text) all my 10,000 or so pictures and (now supported) videos.