Freejacks, or just jacks, are free agents, independents, cargo haulers, scavs, privateers, and generally anyone who doesn't have a greater affiliation.
There are groups and companies that market themselves as freejacks, so through common use the term freejack has also come to include mercenaries for hire. In some places, the term freejack is a derogatory term, but in most appropriately defines those who make their livings on a day-by-day, job-by-job basis.
It's often said "free" represents the lifestyle, not the employment status.
Initially the term came from Jack, being a person who was a jack-of-all-trades: a freelancer or a merc who would do most anything to make a credit. As technology allowed people to roam wherever they want in the universe, more jacks dropped their affiliations and started living free. A true freejack is a person who works completely independently, and are very popular because htye have no political loyalties.
Jobs are place on The Grid advertising work for jacks. These are everything from scav work to bounties, to wetwork, and everything in between.
There are several minor places that list Jack jobs, but there is one major node that gets most of the ads. It's just called The Jacks List, and is very cheap to upload an ad to. A popular feature is its anonymity, but the thing that made it an intergalactic phenomenon is that users can create static accounts that get rated by both parties. So popular employers and employees are quickly obvious. It pays to be careful of anonymous job postings, or posts made by people with few or no ratings.
To some degree, even those with static accounts are largely anonymous, as it's not easy to trace accounts to real people, so this has helped its popularity. Some work harder than others to maintain that anonymity, but the trade has resulted in a few legends in the Freejack world.
Some of the underground Jacklists cater to less savoury, or less legal jobs. There are even some completely private lists. These cover everything from assassination work to opportunities to work for private security firms or armies. There are even those that cater to pirates, helping them organise and mobilise for big raid. There are many who refuse to believe that these nodes exist, holding on to the outdated ideal that pirates are disorganised barbarians.