According to Dungeon Magazine #70, Kingdom of the Ghouls, p. 71:
The god of the ghouls is unknown to outsiders; often called the Lord of Rot and Shadow, or simply the Lord of All Ghouls, many scholars believe the creature is an Abyssal Lord, though others believe it is some form of greater yugoloth or demi-god. In fact, the power answering ghoul prayers is none other than Nerull, the Reaper. Ghouls are the only race whose worship he enjoys; more sensible races worship Nerull merely to placate him.
However, according to later sources, the god of the ghouls is the demigod Doresain. In Dragon #70, Doresain is merely a powerful ghoul.
There are also some humans who worship Nerull, particular the Flan people (of the World of Greyhawk setting), although such worship is not ubiquitous among humans of that world. The sense I get is that a lot of people will make some offerings to Nerull to appease him, but few take him as their primary patron.