One of the fun features of the IRE mud games are their robust player-driven economies. The life of a crafter or tradesman are feasible in Achaea in a rich way that is rare in most other MMORPGs. Making your text-living in one of these ways is, for many people, fun and fulfilling as well as profitable.
In Achaea, most classes have some saleable skills--some more profitable than others. Druids and other "forestal" classes harvest herbs to concoct the curatives that everyone uses. Alchemists gather and refine minerals for the same purpose. Knights (including paladins like my character, Bryony, though she doesn't have the skill yet) forge armour and weapons. Magi create sigils and other enchanted items for various practical purposes. Runewardens and shamans implant runed totems in the ground to protect cities and other property. Even happy-go-lucky jesters can earn a bit of extra money creating bombs for use in combat.
Okay it's not strictly true that Bryony can't forge yet. She can forge...useless wooden clubs. I tried it out anyhow, for fun. Wait, how can you "forge" something made out of wood?!
I don't know of any other MUD where player crafting is such an integral, essential part of the economy. Everyone genuinely relies on other players to get the things they need, even if they just go to player-run shops to buy it all rather than actually bargaining with someone in person. This adds a lot of depth and realism to the game, but the player-base is large enough that everything you need is still widely available.
In addition to the class-based skills, there are also several skills available to anyone who wants to invest the lessons necessary to learn them. For example, you can also train to be a jeweller, tailor, and cook. The license to create your own designs for those skills is a substantial investment, but if you buy one you can use your creative talents to design unique jewellery, clothing, or food. Certain people become well-known for their imaginative and well-written designs, and they are sought out, particularly for special occasions.
In addition, the Gathering and Inkmilling skills together allow you to harvest reagents from the environment or from slaughtered animals and then create inks out of them--inks which are in wide demand for use in magical tattoos or by rune-using classes in combat.
The Gathering skill
Even if you don't want to spend your time harvesting herbs or forging rapiers, and have no talent designing beautiful jewellery and mouth-watering descriptions of foods, the life of a merchant can still be a viable career path. Savvy businesspeople can leverage a little initial capital into substantial profits, by buying or leasing a city shop and using it to re-sell player-made goods, or "imports" sold by denizens on far-off islands and brought back to the mainland.
Bryony doesn't yet have access to any of these skills yet, but I've been thinking longingly of learning Gathering and Forging. Forging is particularly cool in Achaea because each set of armour, or each weapon, has its own unique stats, generated partly at random (though also dependent on the skill of the forger). Most things have mediocre stats, but occasionally you'll get one with really rare, unusually high stats, that you can then sell for a fortune! It's much more interesting (and realistic) than a system where every leather armor you make grants exactly the same bonus.
Different crafting skills appeal to different people (and many people steer clear of that part of the game altogether), but Achaea is the best mmorpg I know if you're interested in craftsman and merchant life.