This's Ambiance Maverick Dark variant, released during Maverick cycle as a WIP, made by Canonical Design Team
I just cherry picked the last bzr commit of lp:light-themes (http://code.launchpad.net/light-themes) in which this variant is still included and made a package.
The theme, as Canonical's Otto Greenslade says, is
... work in progress and *at this stage* is not necessarily intended as a full theme but to be used by certain application environments that would benefit from a darker UI, for example: image/photo manipulation and video editing.
Also, GTK is, for a while in fact, capable of loading a dark variant of the current gtk theme to a specific app. Apps like GIMP, Pitivi, OpenShot or Shotwell may probably benefit from this feature in the next Ubuntu releases.
But this dark-variant is also pretty much suitable for daily use to all dark-theme lovers, so I'm posting it here. It has the usual quirks with GNOME dark-themes, mainly with text entry widgets and it hasn't been updated with the new light-themes changes, such as more contrast in disabled menu items, progress bar improvements, squared windows border, and the fix to menu items slowdown (although I think it's barely noticeable here). But nothing of this is a big show-stopper to its use in a daily basis, so go ahead and download it! ;)
All credits go to the always superb Canonical Design Team http://design.canonical.com
[size=3]Extra[/size]: If you want to give your desktop a little icon contrast, this Faenza Dark iconset (http://gnome-look.org/content/show.php/Faenza+Dark+Icons+%28edited%29?content=132327 blends stunningly great with this Ambiance Maverick-Dark! ;)