Maggie Briggs was a Poltergeist.
Background
In the 1720s, she was a witch and lived in the house of Isaiah Pickett, who - according to the legend - hung her in the backyard for witchcraft. In reality though, she was killed because she was carrying his illegitimate child. Isaiah buried her body in the basement, and her spirit haunted the house ever since.
Season 5
Sam and Dean Winchester are hunting Maggie, when Gary, a teenage witch casts a spell that makes his and Sam's minds switch. Dean and Gary - now in Sam's body - locate Maggie's grave, thanks to some knowledge that Gary has (being a witch himself). They go to the basement of Pickett's house, where they find her grave. Maggie attacks them, but they salt and burn her remains, vanquishing her.