Douma Hostage Massacre
As reported, this was the killing of sixteen civilian men, by agents of the Syrian government, around August 17, 2012, in Douma, just outside Damacus (northeast). However, six bodies found with there with their throats sliced open, have been convincingly matched with six of sixteen captives apparently held by rebel/Free Syrian Army (FSA) forces in Douma. These were allegedly members of the military, and shown on at least one rebel video, perhaps with demands that weren't met in time. (translation needed)
The following is, at the moment, an incomplete assessment of the source evidence.
Breaking the Story
The best visual explanation of this, if a little slow, was made by Syrian Addounia TV. It compares parts of three related videos (see below) and makes the match at the heart of this story. 
The original finding was made by a German amateur investigator and then taken up by Addounia. The investigator ("Urs") has written three articles about it. with the first one started on August 18 with just the found bodies video (later adding the morgue video and the hostage video). From following the posting channel, hrasta1000, Urs already suspected this was a death squad/murder gang that liked to show their own, misattributed, atrocities. Hrasta apparently refers to Harasta, a city next to Douma where the gang seems to be based.  On August 21, she added the finding of the vid with the victims alive. In a later update she mentions to have given the information "to Syria" where it was "received in several forms", linking to this video which is a quite handy two min short version of the Addounia report, doing the matching job. Thesecond article, from August 21, reports the finding of the matching video with the victims alive. The final Addounia TV report gets added in a comment on August 28. The third article, from Sept. 6, makes they case that the gang is still murdering and/or using their old vids for "new massacres" (see below) 
The work involved in similar to, and as good as, that done by staff at Al-Libya TV in February 2011, matching 22 executed "defected soldiers" blamed on the government with nine of those seen in rebel custody, being sentenced to die, according to one transcription. (See here.)
The Video Record
There are four known, relevant videos here.
- The found bodies video: One shows the victims in situ, roughly flopped around to shock the camera with the gaping throats. There are somehow different quality versions of this, differently stamped.  As noted, investigator "Urs" identified Hrasta as refering to the city of Harasta, from with this video death squad apparently operates. The date of death is not clear. The posting date August 17 is an 'at latest' time, and as the bodies are crudely rolled over to revel the open throats and leaf-crusted faces, they exhibit the stiffness of rigor mortis, suggesting that 12 hours or more have passed. Some uploaded versions mention a "Watani Hospital" where the corpses were allegedly dumped.
- Morgue video: Another shows the victims in some sort of makeshift morgue or cooling room, mostly obscured within black body bags.  Regime forces are blamed. Some of the victims, notably #16, but also in glimpses #4 and #8 can be identified here, and are in Addounia's program.
- Main hostage video: The crucial video shows the victims alive, amongst others in rebel custody, apparently being threatened with death. Strangely, it appeared only, as far can be seen, on Aug. 20, three days after the previous videos. The Addounia report cited a video posted by UNITEDNEWS79. This posting can be found, published August 20. http://www.youtube.com/watch?v=p3c9UmypyYI Another known posting is from the same day. http://www.youtube.com/watch?v=BCFj6ltgV4w - title Google translated: "Armor Brigade, DC - Osrbrigad and elements of the Internal Security Forces."
- And finally, there is one earlier video, posted August 14, that could be related (title translates "Armor Brigade capital Bhrsta killed and captured many of the army Asadi 14 8 2012"). لواء درع العاصمة بحرستا مقتل وأسر العديد من الجيش الأسدي 14 20128  This shows the same rebel fighter guarding a row of eight men, some of whom could be the same as those later numbering 16.
The Match Assessed
The six hostages identified by the research in question are given the neck-X here. The matches in question are none of them 100%, but taken together are far too striking to be coincidence. Facial details are not clear enough, especially in the hostage video. But between general body/face/age type and especially distinctive clothing matches, all six victims have arguably or totally matching counterparts among those held by the FSA.
Captive #4 in the Coca-Cola shirt (left/center) is quite distinctive, as is #8, the little guy in blue with possible scoliosis. The best match is #5, with a distinctive long face and two-shirt combination that match in both videos. The weakest matches are the taller men, #6 and 7, but given those laid around them, and their general lumping as those in the right rear corner of the 16, make those calls more than reasonable. The fact that men stood so close together wound up killed together suggests the video was made just before they were portioned off for final execution.
The earlier video, posted Aug. 14, shows only eight captives, most of them different from those shown later.  There are however at least two matches. #5 is just as distinctive here. #10 is likely there in both videos, or someone dressed the same and looking quite similar. #9 looks to be the same person seen here in the same spot, far right, but wearing a different shirt. Next to him is a man in a shirt the color of number 8, but not the same man. There are some others who look similar (like #16 and the guy in position 4 here), but not similar enough to make a match. The scant matches plus the same guard/spokesman in both videos suggests the same captors, of course, and the same basic captive pool. The lack of more matches and the growing number suggest about half of the eight were killed and replaced by about 12 new captives to make 16, before all of those were killed.
Potential Re-Branding of the Same Victims
The third article from Urs1798, from Sept. 6, makes they case that the Harasta gang is still murdering and/or using their old vids for "new massacres" 
Another video, posted September 4 and heavily cited in Urs' third article, shows about five bodies piled helter-skelter on the floor of a morgue room, one on a table, all badly decayed. More bodies are in the cooler in the back of the room, in black bags and looking like the same scene captured in the cooling room. (details forthcoming)
- Evidence of Media Complicity With Terrorists Slitting Throats in SyriaAddounia TV report, published on Youtube, Aug 27, 2012 by Arabi Souri
- Mörderische Milizen aus Harasta-Douma schneiden ihren Geiseln die Kehlen durch #Syrien #Syria – FSA crimes exposedurs1798.wordpress.com, Aug. 18, 2012
- Douma, 16 Gefangene, 16 Leichen in der Kühlkammer. FSA-Crimes exposed #Syriaurs1798.wordpress.com, Aug. 21, 2012
- Nachtrag zu den 16 Opfern aus Douma/Harasta #Syrien #Syriaurs1798.wordpress.com, September 6, 2012
- الشيفونية ذبح المعتقلين من قبل جنود الأسد 17 8 2012Video published on Youtube, Aug 17, 2012 by Hrasta1000
- Syria | Douma | Aug 17, 2012 | Sixteen Victims in Extra Judicial Execution in DoumaVideo published on Youtube, on Aug 17, 2012 by proudlydoumani1
- 14 8 2012"). لواء درع العاصمة بحرستا مقتل وأسر العديد من الجيش الأسدي Published on Aug 14, 2012 by hrasta1000