Dark Mechanicus
Every wonder how Chaos Space Marines get ammo for their Bolters? How they repair their armor when it gets inevitably damaged? Who maintains the Traitor Titan legions? Look no further, because it's all done by the Dark Mechanicus.
As Horus plotted his rebellion, he knew that having the Adeptus Mechanicus would be a major advantage against his father. In an effort to buy them into following him, Horus promised them the Standard Template Constructs in possession of the Auretian Technocracy, and access to the Vaults of Moravec. Infected with Chaos-tainted scrapcode upon opening the Vaults, Fabricator-General Kelbor-Hal declared for Horus, and subsequently slaughtered everyone on Mars who didn't. When the Traitor Legions arrived in the Sol System, Mars was used as their staging point for Terra. Following the end of the Heresy, the Dark Mechanicus continued to function, supplying weapons and technology to the Traitor Legions. In return, powerful Chaos Lords such as Abaddon keep them in line with their shiny hoards of Archaeotech they found during the Great Crusade.
In opposition to the Adeptus Mechanicus, the Dark Mechanicus favors exploring technology to its limits and trying to create new technology. The Dark Mechanicus considers the Omnissiah to be a combined aspect of Chaos Undivided, holding it as a means of progress and innovation. However, it also contains all the drawbacks one would expect of Chaos. Instead of machine-spirits, the Dark Mechanicus uses bound Daemons to provide the intelligence needed to run the machines. In addition, the Dark Mechanicus has techno-organic technology. So everything they make is not only alive, but it wants to kill you. More than usual, we mean.
But then again, if you're a Chaos Space Marine, you're fucking badass and it shouldn't bother you. Member of the lost and the damned, eh....
A good look into the Dark Mechanicus in the fluff is to read Ben Counter's Dark Adeptus book in his Grey Knights series. Seriously, check it out. It will give you a good idea what kind of demented madmen fill the ranks of the dark order.