Blades of Exile

Blades of Exile if the forth of the Exile Trilogy by Spiderweb Software (Jeff Vogel).
It's an RPG where you aren't from Exile (in the first two of the three scenarios anyway) unlike the previous three games.

In the first scenario you must find what is poisoning the water supply.
In the second one there are rebels whom you must either stop or assist. (Your choice.)
In the third you must travel somewhere in a short period of time.

It's the first Exile with a scenario editor.

There have been several scenario contests, the first one run by Spiderweb Software and the others (to date, there have been three total plus, I belive a "short scenario" contest) by fans.

See Also: Exile: Escape From the Pit, Avernum, Riddle of the Spheres

Has a cultish following of people in the usual gamer's demographic (Male, 12-24). There are some older members of the group that most likely bought the game out of nostalgia for its predecessors. These older members are generally highly venerated for their hot ninja scenario-writing skills. Support for the game is now almost entirely shouldered by the so-called Blades of Exile Community.
Also has a small fan-fiction base. Amazingly popular (due to the scenario idea) even though it is technically poor.
Some extra developments are a few arenas, where user-created monsters fight, a good deal of roleplaying attached to the arenas, and a large collection of user-created graphics.