I will give you the pattern, if you agree to send me a large sum of money every time you use it. Deal? OK: You cut out six identical squares of fabric. Sew them into a cube, remembering to leave a little hole so you can invert and stuff that sucker. Then, whipstitch the hole shut. Voila.
Infants find them entertaining to grab, and toddlers find them entertaining to throw and eventually to catch. I usually give a few of different sizes, so they can kind of be stacked in a wobbly sort of way. I put "vocabulary items," mostly animals, on two or three faces of each block, so they're good for a little cognitive work too. Parents like them, in that they look cool, they don't make any noise, and it is really hard to break anything with them.
Advantages to the maker are: 1) it's a good way to use a little scrap fabric and thread, (2) they cost virtually nothing to make, just a few cents for the fiberfill, and (3) they require very little skill or effort, but you still rock the baby shower by having given something handmade. I make 'em in big batches once a year or so, so I have plenty on hand that I can give to acquaintences at work when they have babies. You give something handmade to someone's kid, they pretty much eat out of your hand after that.