I used this tutorial from How About Orange to make "heart pagemarkers". Instead of slipping them into books though, I put them on the edge of a card and created a handmade Valentine. No glue, no scissors, just folded paper, and Voila!
If you're looking for some downloadable goodness, I was fond of these fold and send Valentines from Camile Styles. And over at Love Vs. Design, they have zillions (alright, 60 or so) downloadable products for all kinds of occasions. Then, there's a geometric heart paper set (with envelope lining instructions) over at Creature Comforts.
P.S. There's also a workshop at the National Postal Museum tomorrow!