Google "free adult coloring books" and you're bound to find 1,000 websites offering some sort of black and white swirly thing, but here are three high-quality coloring resources for the dignified scribbler:
Yes, the website is not saying hello to anyone over 12, but they do offer a page of adult coloring sheets with intricate designs that are very similar to those in those trendy big kid coloring books of yore. One page on their site is even called "sophisticated adult picture."
Faber-Castell is an online shop for colored pencils art supplies, but we'll look past this act of blatant self-service because the drawings available are gorgeous. The website offers a nice blend of mandalas, images of nature, and tastefully sexy images of humans.
OK, this site requires you to sign up for a 30-day free trial, but I think it's a top option because it gives you one high-quality drawing for each day of the month. You also don't have to provide any credit card information upfront. Once you sign up, click the "free pages" tab on the top left and follow the prompts to download your first daily drawing.