I have kept many of them, as well as some “if only”-type poetry a married co-worker slipped in books he lent to me (nothing ever happened, and I made it clear that nothing would/could, but there was a bittersweet quality to the method of communication and the poetry itself that was…intoxicating. Whoo. Whole 'nother thread). I also keep birthday cards from my parents and grandparents, and notes from my grade-school friends. Sometimes I wish I had kept more of that stuff - sometimes I wish I had thrown it all away. But I do enjoy re-reading it from time to time.
I used to save all e-mail correspondence from certain people, but in job moves I’ve ended up deleting most of it. I really liked re-reading the lengthy e-mails my boyfriend and I sent to each other during our “courting” phase. It would remind me why I just like the boy so much. Ah, well.