When done properly, shearing is a process that doesn't hurt the animals involved, and can even save them from the pain of carrying excess weight.
But knitters, crocheters, and fiber-lovers alike still owe a big thank-you to...
Alpacas
Merino Sheep
Angora Rabbits
Angora Goats
Not to be confused with their identically-named rabbit brethren, angora goats don't provide angora yarn but rather mohair.
Cashmere Goats
Fancyfancy.
Muskoxen
They're responsible for qiviut, which is one of the nicest yarns you can possibly buy (and the most expensive — I've seen skeins go for nearly $100 apiece because of their warmth, durability, and seeming inability to shrink).