WhatMommyDoes.com offers a wide variety of free, printable adult coloring pages; the link has been re-pinned thousands of times on Pinterest.com, reflecting its popularity. Artist Carlton Hibbert's PatternsForColouring.com is another website that is well-stocked with hundreds of free, printable patterns and images for coloring.
There are other high-profile artists with adult coloring books on the market who offer smaller, free samples of their work online. Emma Farrarons, author of the best-seller "The Mindfulness Coloring Book," has a free download of a page from her book on her blog, UnPetitBlog.com. Another popular adult coloring book illustrator, Johanna Basford, offers up a three-page sample from her recent book "The Enchanted Forest" for coloring fun at JohannaBasford.com.