Zentangle.com – this is the OG, the original and you should definitely check it out. You can get lots of supplies here and project packs. Sign up for their newsletter to stay up to date on new tangles and new products.
Studies and Stories – These are studies and stories that Zentangle.com has collected.
Tangle Patterns – This website has hundreds of tangles and their step-outs organized alphabetically by name.
Tangled by Design 