I'm a semi-snoop. I've looked in medicine cabinets, although not often, and I have no clue why. I like Mal's marbles idea.
(Personally, I keep anything incriminating out of my medicine cabinet!)
I used to be a snoop. I opened Jess's diary a few times when we first started dating. Like Lurkette said, I found out things I didn't want to know. We had a long discussion about it. We're both way more honest with each other in general, and I never feel the need to snoop anymore.