Mike Anderson at Andertoons Cartoon Blog scanned everything in really big so you can print, cut, and assemble any or all of the action valentines or cards from this Super Friends Action Valentine Playbook from 1980. I remember making similar ones with Scott and Randy during freshman year of college. That was in 1981, but it might have been the same book. I remember having a really hard time putting together the Batman and Robin airplane.
I made the Joker valentine this afternoon. I took a picture of it on a piece of office paper so that you can see the finished size. I printed the image on white card stock, used an X-acto knife to cut out the detail (and the slot for the Boom! portion to disappear), and used purple brads to attach the upper and lower arms.
David Friedman at Ironic Sans made several science valentines including this one.
Alex, at Neatorama, made some geeky science valentines including this one.
When Elliot Quince of Quinky Art noticed a gap in the market for valentines featuring reanimated corpses or internal organs being eaten (typical Zombie stuff) he decided to rectify the situation by designing 3 linocut images. High resolution artwork to download and pictures of the process he used to make them are at Quinky Art.
For more geeky valentines see 12 Science Valentines from Mental Floss.
If you are looking for hearts and flowers check out Best Free Valentine Printables.