I'm behind...
Written things:
Old recipe cards hand-written by my mom, grandmother, aunts, great-grandmother, etc. For my 25th birthday, my youngest sister (who was 15 at the time) made me a recipe box with family favorites. Some of the cards are duplicates of ones my mom had so they are written by older relatives and some my sister wrote herself. It warms my heart every time I read one of these recipe cards.
I'm the oldest of four siblings, and I have the letter of love that my father wrote to my mother on the evening that I was born.
I have all kinds of things that DD wrote when she was little, including a packing box from when we were moving from PI to PA where she wrote a whole note about her math skills (in cursive, at the age of almost 6).
I have a small manila envelope that I use for "bills to be paid." I keep it because the original envelope held something that my mom sent to me when we lived in PA, 20+ years ago. It's the last thing that I have with her handwriting on it. I don't know why I keep it, but I do, and keep sticking it back together with clear packing tape when the folds wear thru.