the word "tool" means a system for designing and not a bunch of rulers, pencils etc.
Zentangle is much like "McTavishing" that Karen McTavishing is doing on the long arm. It is a system for designing primarily background designs for machine or hand quilting. Primarily machine. The technique is beginning to turn up in quilt shops in my area as the Zentangle headquarters is very near here. They have retreats. While it is interesting and I have thought about it, it would require another system to learn and then figure out what to do with what I have learned. Long arm quilters are begininning to use it.