I'm a pretty big fan of doodling, but my stuff has nothing on Peter Deligdisch.
His YouTube channel is full of videos where he documents his geometric, abstract doodles. He doesn't offer tutorials exactly, but he does let viewers into his process.
For those who can't draw more than a stick figure but want to feel like they have Deligdisch's talent, he's releasing a coloring book this summer full of his complex, detailed work. You can learn more about the book in this video: