SILVER GHOSTS

The Silver Ghosts are an highly advanced non-humanoid species. They are most unique in appearance, being large floating spheres of silvery chrome protecting a fragile inner body. They are largely benevolent explorers, concerned more with science and experimentation than conquest. They come from a world devastated by a supernova early in their history; despite being abandoned to the cold of space ("the Heat Sink" as they called it), the Ghosts survived. The Ghosts are often associated with forbidden experiments, as they frequently toy with the laws of nature to advance their own knowledge.
Source: Stephen Baxter, Xeelee Sequence

SPLINE

Splines first evolved as a planetbound sentient aquatic whale-like species but decided around one million years BC to become spaceships. Now they are mile-wide sphere-like structures of hardened fleshsdriven by hyperdrives.They are equiped with organic laser canons.
Some splines carry child-craft inside their womb, which look like s silver cylinder which can spread shining parasol shaped sails.
Source: Stephen Baxter, Xeelee Universe, "Timelike Infinity"

SNOWMEN / SNOWFLAKE

The Snowmen are believed to be among the oldest species of the Milky Way Galaxy. They developed at a time when the Galaxy itself was no more than a disc of smoke, illuminated here and there by hot-spot protostars, when there were scarcely any heavy elements to form planets.
At some point of their evolution, they turned temselves completely toward the philosophy that to record events - and only to record - was the highest calling of life. They took apart their world and rebuilt it as a monstrous storage system. They used all the material at their disposal to freeze as much data as they could in an atificial structure called the Snowflake, making near-optimal use of matter, by recording information right down to the thermodynamic limit set by the background temperature of the Universe. As the universe cools down they can store more data.
The SnowFlake is a regular tetrahedron built around the remains of a black dwarf. It measures over ten million miles along its edges. How it maintains its structure in the gravity well of the star remains unknown. While possessing about the same mass as the Earth, it has been puffed out like candy-floss; filled with struts, threads and whiskers of iron, like delicate scaffolding. It has a fractal architecture with the tetrahedron motif, repeated again and again, on all scales.
How the structure, nearly 10 billion years old endured, as it was built when the star it orbits, while todya three degrees above absolute zero, still shone at about eighteen K is another mystery.
It remains unclear if the Snowmen still exist and what their oriignal form was like. A Dully-glowing globe of purple, miles wide, embedded beneath the planar skin of the SnowFlake is believed ot be what remains of them. As no complete exploration of the Snowflake has ever been done, other such spheres may exist in its structure.
Source: Stephen Baxter, Xeelee Universe, "The Godel Sunflower" in Vacuum Diagrams.

Note: The Watchers from the Marvel Universe have a similar philosophy and may be as old.

SILVER GHOSTS

Planet of Origin: In the Milky Way

The silver ghosts consist of a set of symbiotic organisms inside a super-reflective shell that efficiently maintains internal temperature and reflects radiation. The silver ghosts are therefore able to exist in space without additional vehicles. They are known for their pursuit of physics, particularly their experiments regarding changing physical constants in regions of space.
Speculative litterature postulated that at some point in the future, they could eventually be conquered and nearly wiped out by humans, who would adopt their silver skin technology for their own purposes.
Source: Stephen Baxter "Xeelee Sequence"

SQUEEM

Planet of Origin: In the Milky Way
The squeem are space faring group minded aquatic species. They are not especially intelligent or advanced, and resort to stolen technology from the Xeelee, for instance, the hyperdrive.
Speculative litterature postulates that They could be the first extrasolar intelligence contacted by mankind, and by the time of that contact, they would occupy the solar system and enslave the human race.
Source: Stephen Baxter, Xeelee Sequence

QAX

The Qax species is comprised of very few and long lived individuals which developed an advanced spacefaring civilisation.

"Qax biology is based around chemical hyper-cycles embedded in convection cells. A Qax is millions of such cells arranged on the sea surface in a branching pattern covering several miles. Each cell is as wide as a coffee can lid, languidly bubbling like chocolate with nested eddies down to the microscopic level; they're highly organized, living storms." (Taken from Daily Kos. Thanks!)
Being composed of convection cells, a Qax requires complex life support and can be damaged easily - therefore their contact with other species is usually indirect.
The Qax had a longstanding alliance with the sentient Spline ships who carried them around the galaxy.
Hypothetical Litterature postulated that Circa 5000, the Qax would encounter humanity and a brief war would be fought leading to the occupation of Earth and its colonies by the Qax until they were forced to leave dure to their hommweorld's destruction.
Source: Stephen Baxter, Xeelee Sequence
Thanks to Karen Wehrstein for the great Qax picture

PHOTINO BIRDS

Photino birds are a supersymetric species of dark matter entities that inhabit the gravity wells of stars. With multi-billion year life span and the ability to travel faster than the speed of light. It is theorized that they began their civilization shortly after the birth of the universe and the beginning of stellar evolution. Being dependant upon stars for their breeding grounds, when the first stars of the universe began showing instabilities such as supernovae, they set out to correct these instabilities by accelerating steller evolution throughout the universe. The end result would be a stable universe dotted with white dwarfs. This make life impossible for most planetbound life forms. Upon discovering them, the Xeelee species, one of the most advanced in the universe, started fighting them until they realised this war to be unwinable and decided instead to start moving to other universes.The Photino birds hardly know about other existing life-forms with the exception of the Xeelee.
To fight the Xeelee, the photino birds have developped means to use galaxies as weapons and to propel them to near lightspeed. Photino Birds share the awareness and memories of their parents and ancestry, as each bird is actually an exact copy of its parent
Source: Stephen Baxter, Xeelee Sequence.

XEELEE

The Xeelee are a hyper-advanced civilization which has existed for billions of years whose technology far exceeds that of almost any other known species. They are spread across the universe, typically being concentrated in the heart of galaxies whose black holes they use.
They have also spread gigantic artifacts and huge inscrutable structures throughout space. The most notable of those being the Ring, AKA The Great Attractor, a huge construct drawing in galaxies for millions of light years around with its awesome gravitational well. The Ring, devised to tear a hole in the universe and open a portal to parallel universes, is to be used as an "escape hatch", if the Xeelee were to end up being unable to stop the Photino Birds from aging the stars of the universe.
Hypothetical letterature speculates that Humanity might in the future encounter the Xeelee and begin to war with the Xeelee, which is most likely to last over ten thousand years and to be lost by humanity in the end.
Source: Stephen Baxter, Xeelee Sequence.