[Kereth]

A few examples of MUDs that spawned this context are in order then. We'll take two of the more popular ones that I have had experience with: NewWorlds and Discworld.

Discworld is just flat "heartbeat exp." Basically, you slowly tick exp as long as you're logged in. It's not nearly as fast as farming, particularly at later levels, but it's enough that it makes it worth your while to sit and idle, and yes, you don't have to do anything at all, not chatting or moving around. If you're logged in, you get heartbeat exp. I think the game will kick you if you idle too long though, probably as a control, and triggers and scripts are forbidden there, so using something to keep you idle and logged in would be cheating.

NewWorlds, on the other hand, gives exp and cash out in chunks every set number of hours spent logged in. It's a lot slower accumulation though, compared to Discworld. They call this the "Roleplaying bonus." They also have an "mia" command that lets you stay connected, and their multiplaying rules permit two characters to be owned by a single player and logged in at the same time (with certain restrictions).

Both are fairly large muds (by reported player base size, though of course, that has to be taken in stride with the effects of idle exp). You can track the basic information from the listing on here, I'm sure.