This website is geared toward kids and has simple black and white drawings to color that cover a number of kid-friendly themes that would appeal to coloring fans of a variety of ages. For example, if you click on the "cartoon" section, you'll find kids' favorite characters from Dora the Explorer to Monster High. There are some great educational sections, too, covering topics like the alphabet, numbers, animals, shapes, and the flags of countries from around the world.
In addition to being fun for kids and parents, this site could be a useful resource for teachers, day care providers, babysitters, and more. You can sit down with a child and let him or her pick out their favorite sheets to color, or you can print out a selection yourself and surprise your little friend. This would also be a great site to bring up when you visit the public library if your library (like my local branch) lets you do some free printing.
It's nice that the address for the site is so easy to remember, too. I'm pretty sure that if you tell any kids you know about 123 Coloring Pages, they'll be asking for it by name. :) And by the way, these can be fun for adults, too--the section of coloring pages under the heading of "landscapes" has a lot of detailed pages that adults would enjoy, with imagery drawn from cultures around the world.