While perusing the wonderful art at the Zentangle flickr group I noticed that someone had used one of the tangle patterns I haven't posted yet. So I figured I'd better post it, lol!
It's one of my favorites. It's a great pattern for learning more about shading--try increasing and decreasing the amount of dark. Add more than the two lines. Leave out the first two steps (the grid) and just start drawing boxes. See how the look changes when you do these things.
It's also a great tangle for learning how to curve your pattern, and that's why I added the grid. When you draw the first two steps try curving the lines. Then draw your boxes following the curves.