Space is large and remote, with long stretches of isolated nothing. These distant waypoints and astral lacuna form perfect ambush points for the many pirates that stalk the galaxy. A spaceship is a tempting prize in and of itself; as rip drives and other ftl devices are expensive to make. Any cargo that happens to seized is a bonus. Rarely, pirates will raid independent planets and freeholds, but the logistics of transporting their booty out of gravity wells is beyond most pirates. In addition, most freeholds are themselves populated largely by pirates, making them less than soft targets.
Pirates form a huge section of the Galactic underclass; those desperate losers looking for an easy score to get rich quick. Independent pirates make up the bottom of the barrel in pirate society. They use scrap ships and make shift weapons, and can present a real danger to the unwary, but are no real threat to hardened targets. Above the independents are the privateers, state sanctioned pirates. Many polities empower their more heavily armed citizens to take action against the state's enemies. Currently with the rise of the Agredeem empire, the Seeders and other allied powers are licensing many more privateers to disrupt the God Emperor's trade routes. Also, many polities employ privateers as pirate hunters. There is a notable convergence in attitude, culture and personnel between pirates and pirate hunters.
Aside from privateers, there are some pirate polities, that is, states that use their armed forces to commit piracy. The Republic of Sea Serpents is notorious for this practice. The Joker merchant marine corps spends as much time waiting for victims as they do protecting Sea Serpent shipping interests. According to the Sea Serpents, the Jokers are private contractors and they can not be held liable for their actions on their own time. Most states find believing this more convenient than contesting it.
While not strictly speaking a polity in its own right, the leading edge of the migration of the Empire of Friends is a refuge for all the outcasts and criminals of the empire. While the so called "pirate corona" is a poorly policed zone, it is not ungoverned. The corona has its own twisted version of the Creed of Friendship, and is ruled by several strong factions of ner-do-wells, including the Bad Dude Alliance and the Coven Collective. As the empire, and thus the corona migrate, independent pirate bands are absorbed by these factions. They are organized enough to pull off daring planetary raids. There is a constant struggle between the main stream empire and the pirate corona, but there are also back channels that let the two sides coordinate against outsiders.
The Machine lifeforms of the Xietentoeten Economic Zone are strongly mecha-chauvinist, and frequently do not respect the property rights of organic life. Armed parties of XEZ will target and rob non machine life just because they can get away with it.
Two groups of sapients follow the Candymen. The Intergalactic Candy Company is the more respectable, business side of the candy. The Candy Cults are the more wild and primal side. While the Candy Company serves the Candy for material gain, the Candy Cults serve through faith and addiction. Made up of Candylings, Candy Mutants and assorted candy addicts, they rove the stars, looking for tribute for their candy masters. They have a reputation for mercy. Instead of spacing captives, they keep them alive. They accept ransoms to rescue high profile captives, and will release others at the nearest freehold, but not before exposing them to candy narcotics. Many take to piracy themselves to feed their new addictions.
But of all the pirate polities the most feared are the Reborn of the Undersphere. Hailing from a cluster of planets below the Galactic plane, Reborn corsairs raid much of the Galaxy. They don't just seek ships and goods, but personnel as well. Reborn are created through a mixture of mental conditioning and a certain retro virus. The process of creating a Reborn is fraught with peril. Roughly one in fifty successfully survive the process; the rest are turned into non-sapient creeps or killed outright. Even other pirates fear the Reborn.
Dream piracy is the act of psychically entering another's dream with ill intent. It is most often practiced by the Pirate Corona and the reborn corsairs, as well as Father Corp. The first two factions use this technique to find hidden intel about potential targets, as well as to subvert and suborn the minds of defenders. Father Corp steals entire dreams wholesale, leaving the victim dazed and listless until they can have a proper dream.