http://www.zentangle.com/index.php I think this is the website of the people who originally started marketing this idea.
On a side note, my daughter had an art teacher whose preferred medium was water color and oil pastels. She almost always filled in her empty spaces in her paintings with small patterns. It really reminded me of this whole Zentangle thing....
http://beezinthebelfry.blogspot.com/ This is the address to the blog of the author of the book I got.
And this is where I first heard about Zentangles.... http://nayswonderland.blogspot.com/2011/02/what-is-zentangle.html (don't you just love her hat?) :)