I saw a Boggart in my house not very long ago. He was crouching on the top of a set of shelves in the bedroom as I approached the door. He saw me and quickly slipped down to the floor and went to hide. He was only little, probably only two foot tall, and was a skinny, old looking little Boggart, who seemed harmless, and I didn't feel surprised to see him there.
There was one of the creatures near my parents' house, possibly homeless as his old house had been knocked down. This was a half-timbered hall where my Grandmother was a servant; she said every night the Boggart used to fill up any buckets left out by the well.
The oddest thing I experienced, which had little to do with creatures or beings from the underworld, was that as my grandfather died, he knocked on the window of my parents' house, which he had been born in. My father was with him as he died, and when we later told him about the knock on the window he calmly said that it was Grandad, and that it always happened when somebody died in the family.
