"Memento Mori", Latin for "remember your mortality" or "remember you will die", is the practice of reflecting on one's inevitable death as a reminder to live life to the fulluest or, in staying with the Latin theme, "carpe diem".
This site is inspired by a blog post I once read describing a U.S. Navy solider who carried around a Memento Mori sheet full of small squares, each representing one week of his life. At the end of the week he'd fill in a square, creating a graphic representation of his lifetime. I tried this myself. The picture is a powerful reminder of how little time we have, and how important it is to sieze the day.