In What is the best kind of evidence of afterlife I wrote that EVP is one of the more interesting areas of research, though not one widely discussed, for some reason.
The short explanation of what EVP is – these are recordings, usually audio, of voices that seemingly couldn’t be produced by any means other than paranormal. Thus it’s named “Electronic Voice Phenomena”. The stronger evidence is provided by the recording of messages relevant to those who hear it, like it would be through a human medium. There appear to be cases of visual EVP as well, such as recordings on VCRs and such.
One of the more prominent people who researched EVP was the Latvian-born Dr. Konstantin Raudive. He recorded thousands of EVP messages and, after he passed, was the messenger to his fellow researchers in their EVP experiments.
There’s also a good overview of the history of EVP at the Anomalist.