Well, Second-Edition-wise, the neogi are all over arcane space. They were a pretty central Spelljammer villain, as those things went.
Third-Edition-wise, they're in Monster Manual II. I've used one since the book came out.. immediately after the book was released, since I'm a fan of the creatures.
The PCs were searching for some missing women (for rather convoluted reasons), and eventually tracked the bandits who'd kidnapped them back to an old fort in the wilderness (the Merithian Fortress, from Eden Games' Wonders out of Time).
I'd added another basement to the fortress, within which dwelled the neogi who'd dominated the bandit chieftain. The neogi also had the women down there, and was using them as its servants. He also had some abyssal maws on loan, so to speak. The PCs of course slew the neogi and the maws and rescued the women.. but they were certainly a bit unsettled by the neogi, especially with his mind-control tricksies.
I was going to use the race as a big villain race, but didn't.. but if the PCs go back to that area, they'll find more obnoxious neogi causing trouble, perhaps...