Postcards from places I visit, or friends/family have visited.
I like ones with a picture of the specific landmark, and no captions or collages of pictures.
I write where I was, who I was with, what we/I were doing, and when I was there on the back.
I keep them pinned to my wall at uni at the moment but plan to put them into a photo album eventually, as a scrapbook of memories.
I started a few years ago with cards from Rome, and I have some from Hadrian's Wall, Warwick Castle, Isle of Wight (various places on the island) and Canterbury Cathedral and some more obscure locations.