I never took dirty ones, but I of course rifled through my mom and stepdad's drawers whenever I was at their house alone, as they always had magazines, porno tapes, and toys around. Once or twice, I took my stepdad's (clean) bikini underwear and jerked off with them.
Outside of family, I used to have a friend in high school whose house our friends used to hang out at all the time because her mom just didn't give a shit (in a good way). She had an older cousin who stayed there a lot - he was a huge guy, very tall and broad, and I thought he was kind of cute. I managed to snag a pair of his tighty-whities (again, not dirty). I was turned on because they were so big compared to me.