The Mutara Nebula
The term Mutara-class nebula refers to any nebula with similar properties to the Mutara Nebula.
In 2374, the USS Voyager came upon a radioactive Mutara-class nebula. The nebula emitted subnucleonic radiation which greatly affected humanoid skin. The nebula was about 110 light years long and would take one month to travel through. The Voyager crew (except The Doctor and Seven of Nine) were put in stasis pods for the duration of the trip. During the trip, the ship suffered many problems due to the radiation including gel pack randomization problems and computer errors. (VOY: "One")
A Mutara class nebula in Devore space.
In 2375, the USS Voyager was carrying telepathic refugees through Devore space, with the intention of meeting up with a transport ship that would take the refugees to safety. This ship continually gave them new coordinates, one of which was a Mutara-class nebula. It was later discovered that the transport ship had been intercepted, and at least one dozen ships were waiting inside the nebula for further ships to fall into the trap. Voyager did not go to the nebula. (VOY: "Counterpoint")
In 2378, the Doctor took holo-images of a Mutara-class nebula while he piloted the Delta Flyer. When the shuttle later encountered a subspace eddy, the Doctor suggested that it was probably just the gravimetric shear coming from the nebula. (VOY: "Renaissance Man")