Dover Publications, a popular and longstanding children's coloring book company, offers a free children's book sampler by email that includes printable coloring sheets, puzzles and other fun activities. You can sign up for this weekly offer on the company's website.
Crayola also offers a number of free printable coloring sheets on its website. The website organizes sheets by category and features a number of popular children's characters, interesting designs, plants and animals.
The website Coloring-Book.com also has a vast catalog of coloring sheets available. These feature characters and settings from popular books, movies and television shows, as well as holiday themes.