Coloring.com is the site to go to when your kid is bored with the colouring games on all the other sites you have been regularly visiting. The layout may not be promising but they have a multitude of choices, grouped by theme.
The line drawings are beautiful, you get more range of colours than at most other sites, and there are even a few textures to add.
If your net speed is low, you may find a little delay in colour picking as well as colouring.
Once done, the site generates a code meant for your 'my space' profile - but which can be used elsewhere as well :)
The url - http://www.coloring.com/pictures/choose.cdc
P.S. You can register at the site, though it is not essential. To do so, use the url - http://www.coloring.com