Some years ago, I shared a split-level house with another guy. He had a nice looking girlfriend and after a few times of him having her over on the weekends, I soon realized that she would usually spend the night with him and they would often have sex before going to sleep. Since my roommate basically lived upstairs, and my bedroom was downstairs below them, I could always hear what they were doing. During sex, my roommate was always very quite but his girlfriend was quite vocal and I could often hear her moans and squeals. This quickly got to be a turn on for me knowing that sooner or later that night they were going to be having sex and I would be able to masturbate while hearing her whimper and moan as he drove his cock inside her.
As these visits soon got more frequent, I would always be waiting for them to go to his room for the night. I would strip off my pants, put on a pair of her panties that I had acquired, and wait for them to start having sex. I would then quietly climb the stairs and sit in the hall and start pulling on my cock as I listened to them. I did a lot of edging trying to hold off until I could hear she was going to cum. When she did - I would let myself cum too making one heck of a puddle on the hardwood floor.
A few years later my roommate moved out but I still have a nice collection of memories (and panties).