Also known as nonograms, logigraphs, Hanjie, griddlers.

These are great! I could do them all day. I usually do them digitally but they are essentially a pen-and-paper puzzle. There are loads of free ones and some good commercial videogame versions too.

See also:

• Mario's Picross
• Sky Castle - Android: Free app with an emphasis on smaller puzzles that are grouped to form larger pictures.