The final major type of economy is the player-based economy. This sort of economy embodies capitalism to the extreme. Most of a players armor, weapons, and other gear come from other player making them. This system offers a large basis for a community. If you get a really good gun from a crafter, you will remember them when you need another weapon. If your armor server you well, you will remember the armorsmith when your friends need gear.
The old version of Star Wars Galaxies shows this economy well. On day 1 (for the lucky few who got to log on), they spawned into a world that had no weapons except those they spawned with, and absolutely no armor. The players made do. Crafters made their own tools and slowly cobbled together armor and weapons, and sold them. The hunters then could kill bigger things and bring back loot components to make better gear. The cycle continued until there were regular hunters and crafters working together to make really amazing gear. The bare worlds that were spotted with tiny NPC outposts were soon paved with player cities and harvesters.
The community factor of the original version of Star Wars Galaxies was impressive. The forums and in-game dialogues were the epitome of what an MMORPG should be (to me at least). You could buy armor and weapons without talking to anyone, but you could usually get a much better deal and a crafter to meet your specific needs if you just asked.
There is a downside to this economy type. It is slow to start. Sony had to set up a vendor to sell tracking droids a few months into the game, so that the bounty hunter profession could do their missions because droid engineers hadn't gotten to the point where they could mass produce the droids. To make the first armors and weapons, players had to experiment and the initial development was expensive (relative the amount of economy). However, those who made the first sets of bone armor sold them for a good profit. The first architects made a killing in the market in the early days.
The economy is good for a MMORPG that is just starting, and will have many new features added later. With Star Wars Galaxies, mounts and vehicles hadn't been added yet, so players didn't have means to travel far from civilization unless they walked. That meant that they were fighting lower level mobs, and needed lower level equipment. As the game develops, the economy develops. In the end, the economy is an amazing addition to the game, and the community that goes with it is even better.