Don't know if there's a thread already, but I was just wondering. I keep the letters my "mom" sends me. I've kept that anonymous letter the supposed to be mayor sent me on day one. I've also kept some letters from my favorite villagers. What about you?
The moving away letters that include villager pics, I don't display all my villager pics so the letters make good storage and help me keep track of which pics are which. I also keep the ones I find interesting or funny, like this one Lopez sent my guy character recommending clothes and there was a skirt inside.
Generally everything that's not from the Happy Home Showcase/Happy Room Academy and catalog purchases, as well as ones that I feel like I've saved before.
I will eventually have to clear out space in my NL file. I will be keeping goodbye letters and "mom" letters for sure.
I throw away all "Mom" letters, because my real mom is deceased. :'(
I keep villagers letters whom have moved away in neat folders - Kidd, Kody and Lily, as well as a few others from current villagers that are funny and adorable.
Also musical letters from my real friends who have visited my town and sent me letters. And my boyfriend lives in my town, him and I exchange letters too. I keep those.