Space Nomads: Several kinds, ranging from perpetual wanderers (say, the remnants of the londian), through long-range explorers (say, the spinbrights), to people like the residents of Imperial city-ships, skymalls, empire ships and other large starships, some of which travel with a purpose, and some of which just stuck an engine on their habitat so they could enjoy a change of view from time to time.
Racial Remnant: Most notably, the trilateral londian, who are also an unusual case because while the modern Londian Wanderers are clearly descendants, both genetically and culturally, of the elder-race of the same name that used to dominate the Londian Traverse, they don’t know exactly how they ended up that way. Their ancestors, as they tell it, were off-world during the collapse of the Londian civilization, and stayed that way afterwards… and have no idea why the collapse happened. Their technology has regressed somewhat from that of the planet-bound londian, and now seems oddly arrested, even given the constraints on their ship-bound society.
You can bet archaeologists are looking into that one pretty closely.
The Mothership: In the military sense, the hyperdreadnoughts, the directional Supremacy-class bases, and probably most of the other classes that qualify as The Battlestar, or just plain old carrier classes, and fleet carriers. Also their obvious civilian counterparts, the fleet carrier equivalents that ferry freighters and other civilian vessels around relativistically that don’t themselves have relativistic capability.
In a pure-mass sense, a large number of wandering city-ships, both Imperial and those used by nomadic species such as the londian, and in a specifically Imperial sense, the Empire Ships – essentially, flying city-states-cum-endless-parties whose job is to travel endlessly looped courses with one end in the Imperial Core passing through the hundreds of ecumenical colonies and exclaves out in the far reaches of the stargate plexus to keep the culture thoroughly mixed and stirred, and not turning into anything weird out there in the fringes.
And, of course, the Embassy Ships and the All Good Things skymalls, which are similar, except act as cultural
bombs ambassadors to foreign star nations, where they turn up for memetic warfare goodwill visits, spreading intimidation peace and good will, their decadent ways happiness, propaganda cultural interchange, ethically bankrupt perversions of science technological advancement, and the wonders of hypercapitalist exploitation free trade and disobedience, insolence, selfishness, libertinism, etc., etc. actual freedom to everyone.
Generation Ships: The Empire itself has never used generation ships, despite thinking of the concept. Regular lighthuggers don’t count – the whole immortality thing means that while children may well be born en voyage, the people who got on will by and large be the same people who’ll get off. Their first burst of interstellar colonization (the Thirteen Colonies) was done subluminally, but the Deep Stars carried frozen colonists, so they don’t count either. And the wandering city-ships don’t count simply because they’re not a means of transport, they’re places people live – and people come and go all the time, any time they pass through a system.
Which is not, of course, to say that the concept’s never been used by anyone else; I’m sure it has, probably quite a bit, and still is in places beyond the Associated Worlds, or where they’re still en route. And, arguably, the couple of nomadic cultures (the londian, for one example) who live on wandering city-ships and have nowhere to leave them for but other city-ships might well count.