It has been a goal of mine to own a Mechanicus Army ever since I first read about them during my early days of playing 40k. I have admired many converted Mechanicus Armies over the years and owning one of my own has been out of my grasp until now. My desire has been reinvigorated with the recent release of Forge World's Imperial Armour: Horus Heresy series of books. I'm hoping for a slow but steady release schedule so it won't strain my hobby budget too much. I just wanted to list a brief summary of a few of the units with links incase you wanted more information on them.
The Skitarii, also known as the Tech-Guard, are the cybernetic military forces of the Adeptus Mechanicus. They are used to defend Mechanicus Forge Worlds, manufacturing installations elsewhere in the Imperium and often accompany high-ranking Tech-Priests and Magi as their bodyguards. Skitarii also serve as the armed forces that defend the Mechanicus' Explorator vessels that explore the galactic frontier. Skitarii safeguard Titans on campaign from boarding actions and prevent hostile forces from reaching the war engines' dead zone, where its main weapon batteries are useless. A number of Titan Legions maintain their own Skitarii detachments for this purpose, and if they do not, their Mechanicus masters will provide ample forces to protect the God Machines. The term "Skitarii" is formally applied by the Tech-priests of the Mechanicus to all of the military forces under their command, including infantry and armoured vehicles, save for the Titan Legions and the Knights. However, the term colloquially has often referred solely to the Adeptus Mechanicus' cybernetic infantrymen. http://warhammer40k.wikia.com/wiki/Skitarii
As the heavily augmented cyborg shock-troops of the Ordo Reductor faction of the Mechanicum, the Thallax differed from the Skitarii regiments both in their purpose and the unique degree to which their bodies had been reshaped by augmetics. The Lorica Thallax which encases them was developed from the same Power Armour technology which led to the development of Legiones Astartes battle plate, but taken down along an entirely different avenue of development. Requiring a robust human specimen, the Lorica encased the major organs, nervous system and cerebrum, but replaced the skeleton and limbs entirely with armoured mechanical systems powered by an internal reactor-core. The agony of this process, along with the replacement of the usual human sensory apparatus, proved so traumatic as to require the surgical excision of the pain centres and emotions, leaving the Thallax a cold, calculating killing machine, yet one that still retained a degree of independent human thought. However, for some within the Mechanicum this transformation of the human mind skirted the edge of abomination such as that posed by sentient "Abominable Intelligence" (A.I.). Given the resources required, the difficulty of creating a Thallax and the fact that an individual's will to live was a factor in whether a usable measure of sanity was retained in the subject, Thallax conversion was often reserved for suitable Forge Guard and Skitarii fallen in battle and subjects obtained from more mysterious sources by the unfathomable and baleful Ordo Reductor. http://warhammer40k.wikia.com/wiki/Thallax_Cohort
Other Forces of the Adeptus Mechanicus Militant
Forge World Mechanicus Models
The character at the top is Magus Dominus. The dreadnought sized unit pictured with the Thallax is called a Castellax. No info that I can find on them as of yet.