It's actually free for unlimited use if you run your own network controller, see docs in /controller and /service on GitHub. There's now a 100 device free limit on my.zerotier.com which is just our hosted SaaS network controller.

(I'm the founder of ZeroTier)

actually the site/documentation hasn't been updated but it's now free for 100 devices (with a price increase beyond).

from an email earlier this week:

"For new subscribers we've increased the price to $29.00/month (USD), but we've also increased the maximum number of devices for free accounts to 100.

We think this better reflects the difference between personal and business users, providing plenty of devices in our free version and charging enough for our business customers to justify adding new features and services and to support the continued development of the entire ZeroTier ecosystem."