"They call themselves the Nomads"
Other races prefer more colorful names -- batteries, bourgeoisie, drifters, ingots, wedges, tubers; the list goes on.
There are traders who deal with the Nomads their whole lives and never even see anything beyond the Fleet. These are becoming increasingly less common as the Nomads creep across space. Admirals, CEOs, and eccentrics alike have paid billions of their currency for Nomad machines, but for a live Nomad to operate them, planetoids, mines, small countries, and the like have all been traded over. First, one thing should be established. Nomads build things for other people to use. However, every good large-scale piece of equipment, from war mechs to space ferries to biospheres, has a little circular hatch in it. "Little" means about 1.8 meters in diameter.
Here's why. This, as far as anyone knows, is a Nomad:
A silvery canister with unexplained markings all over it.
The Nomads are, of course, a wandering race of mercenaries and traders. The rest of the civilized galaxy knows a few things about them. For example, while they're apparently content to sit in large racks on trade shuttles, the Fleet nucleates around small groups of recreational vessels. They also seem to enjoy formal ceremonies generally attributed to primitive post-Iron Age cultures. And while they do sell technology, it only comes with instructions. With the exception of sheer genius applied to the task of reverse-engineering, Nomad products are only manufactured by Nomad traders.
Another oddity is the fact that Nomads have no use for the three gods -- visitors to their ships will find the occasional ViS emblem, but "the respect they, in their giant walking suits, show to Subspace is not mystical at all... indeed, while they are distinctly neutral in most things, they show a sort of familiarity towards the Masters of Efficiency."
[1] When they stop on planets, they are almost always those that follow the ViS, but nobody really knows why. Several ships touch down, mechs in the shape of the natives come out, they ask vague questions, and they leave.
Obviously, a OMYL race.