This is the most common job description for adventuring types - in fact, almost all will be mercenaries of one sort or another, though of course not all will be the gun totin' Soldier-of-Fortune type. Strictly speaking, anyone who performs a service primarily for money (or some other personal advantage) is a mercenary.
The classic mercenary - the military "adviser" - will seldom be out of work. Although the public perception of the Oikumene is that it is in an age of peace and prosperity, in fact there are constantly small brushfire wars happening in the outer reaches and beyond. Disgruntled fanatic groups migrate out beyond the fringes of the Oikumene where they can put into action their own particular plan for utopia, and usually discover that some of them have slightly different ideas than some of the others. In most cases, this discovery results in the usual run of bloody purges and savage repression, but sometimes the split is too evenly matched for this simple solution to work, and civil war eventuates. Mercenary paradise.
Most military mercenaries work as a team, sharing resources - this becomes important when everything goes wrong, it's time to get out, and you need a ship.
Another lucrative field for the mercenary is in corporate and personal security. This tends to be the field of choice for the individual mercenary, though opportunities exist for teams as well.
Pirates, when it comes right down to it, are just mercenaries without a steady job. Or mercenaries are fully-employed pirates, whichever. Piracy is an uncertain profession, though it can be lucrative if the luck goes with you. The key to being an effective pirate - apart from overwhelming firepower - is good intelligence. There's nothing more frustrating than running down and boarding a freighter only to discover that all it's carrying is a cargo of corpses destined for stellar cremation. You've then risked your life, made yourself liable for summary execution, and all for a couple of thousand stiffs.
Truckers of the spaceways, carrying passengers and products from planet to planet, free traders form the web which holds the Oikumene together as an economic entity. While most large corporations maintain their own fleets, the owner-operator or small freight company is still the most common carrier of people and stuff from one point of the galaxy to another.
The key to making a living as a free trader lies in planning a route which minimizes the time your holds stand empty, and which will allow you to maximize the profit margin on any given cargo. Route juggling is a fine art, and with the slowness of intersystem communications, a certain amount of gambling is required since it is seldom possible to guarantee that the cargo you think is so precious will actually be as desirable when you get to your destination. Rumours of a lucrative market can cause a freight-rush which swamps the market and leaves late-comers with worthless cargoes and no means of meeting the expenses of the run. Not surprisingly, free traders tend to be highly competitive and secretive, though they will often band together against outsiders.
The relatively ready availability of space travel makes it very easy for a criminal who is not actually caught in the act to get away. While some planetary governments may maintain their own official inter-system enforcement personnel, jurisdiction issues can complicate the apprehension of a wanted criminal. Most government law enforcement agencies therefore find it easier (and cheaper) to rely on one or more non-partisan organisations to find and arrest those villains who have escaped off-planet.
The three major organisations in operation are (in order of size and respectability) the Intersystem Suspect Apprehension Corps (ISAC), Interstellar Investigations Inc. (III), and the rather shadowy ScumTrackers. These three hold virtually all of the interstellar investigations market between them and maintain their own offices and personnel on most major planets. They in turn make extensive use of local investigators to locate the suspect, but handle the arrest and all bureaucratic matters regarding extradition themselves for their clients. Only ScumTrackers will handle perpetrator terminations as a matter of course, the other two will make every effort to return the offender for execution and are supposed to kill only in self-defence, and then only in accordance with local law.
None of these organizations have any sort of automatic mandate and must negotiate with each planetary government individually for jurisdictional rights. In most cases, this requires little more than an official request for recognition, but cases exist where ISAC, III and (especially) Scumtrackers are distinctly unwelcome. Even on planets where the agencies are officially recognised, local enforcement officers can be uncooperative and obstructive if they feel that their perquisites are be pre-empted by outsiders - often, the job of the local Agency Chief is largely political, and consists of massaging the wounded sensibilities of the local law.
Planetary law enforcement agencies, and even individual law enforcement officers, may apply for affiliation with ISAC if they do not have a local office handy. Affiliation benefits both parties; it allows the local agency to access information from ISAC databases and to share in the commission for a successful apprehension on their territory, and ISAC gains a de-facto office on the planet without the expense of maintaining one. ISAC standards are high, however: affiliated officers are required to undergo in-depth training in interstellar law as well as becoming familiar with the legal peculiarities of all of the ninety inhabited worlds of the Oikumene. If in pursuit of an interstellar fugitive, affiliated officers are entitled to call on local offices of ISAC for logistical support and backup at need. Affiliates are not permitted to take part in investigations which do not concern their own planetary jurisdiction, except as observers for training purposes.
Bounty hunting is a dangerous business. The targets are seldom willing to come quietly, and the exigencies of the work often place the bounty hunter outside the local law as well. Most bounty hunting takes place under the aegis of one of the agencies mentioned above, but freelance individuals also work in this area.
Most commonly, espionage is commercially motivated, though as noted above there is enough military action going on all the time to keep spies employed in that area as well.
In a milieu where space is effectively limitless, and space travel, if not actually cheap, is at least within the reach of a good percentage of the population, the field of real estate branches out beyond merely selling houses and office space, and prospecting becomes more than a matter of sluicing gold out of a river-bed.
Resource locators are those individuals whose profession it is to physically search out habitable planets, or other sites of interest such as valuable mineral deposits, Precursor ruins, derelict ships for salvage, and so forth. Most locators are employed by large commercial concerns, but a few are sponsored by educational or conservation institutions, and fewer still are freelancers with their own equipment. The potential rewards are great, but operating a starship - even a very basic starship - isn't cheap, and the returns are uncertain at best.
A locator needs a fast ship with broad sensor capabilities. Cargo space is not a high priority; nor are luxurious accommodations. Robots of various types are essential, since most of the work is done far from any human aid. Locators must be knowledgeable in an enormous range of fields - astrogation skills, planetary sciences, mineralogy, cybernetics, physical and biological chemistry, astrophysics - all these and more. It also helps to be slightly crazy. Most locators get very, very good at Solitaire.
The payment to the locators varies. Some are on salary, but most are on small retainers with the bulk of their income coming (hopefully) from commissions on the resources found. The size of both the retainer and the commission varies from employer to employer, and is largely the result of the bargaining skill of the locator, though a locator with a proven track record can naturally command a higher rate of compensation than a rank beginner.
The Patrol are the neutral military force sponsored by payments from all of the member planets of the Oikumene. It's function is primarily to stamp out piracy and thus facilitate trade between the member systems, but it is also available for use as a deterrent against interplanetary adventurism.