Zerotier is an alternative to the higly popular service, Hamachi.
Hamachi and Zerotier are both essentially the same service on paper.
Both are essentially just P2P VPN's, both with paid and free plans available, albeit while targeting much different users.
As a GaMeR, I like to use a P2P VPN for things like locally hosting servers for things like Minecraft, Teamspeak, Mumble, etc.
At some point a few months ago (as of writing this), we even used it to share our computer's combined power to mine Monero, even with my extremely limited programming skills.
Oooooh technical jargon time.
I love to confuse everyone with this part.
A P2P VPN is actually called a "Peer to Peer Virtual Private Network".
Normally a VPN is used to trick some software into believing you are in another part of the world than where you actually are, usually for things like Netflix.
A Peer to Peer VPN, however, essentially adds another step, replacing the location part with what is essentially a router that connects the users together, as if they were on the same internet as each other, while being almost entirely disconnected from each other.
If you have lots of extra money to spend, you can throw money at third party servers to host your stuff, and connect to them through an app or something, or you can use a Peer to Peer VPN to do essentially the same stuff for free.
Here is where I will explain why I don't particularly like Hamachi, even though there is much more support for it.
Hamachi supports up to 5 users for free, until ramping up significantly to $49 USD a year for 32 users ($1.53 per user), and $200 USD for 256 users ($0.78 per user).
This pricing is abysmal, and for what I use a P2P VPN for, I would be required to pay $50 a year to support the 9 current devices (remember, Raspberry Pis are included here), which just so happens to be $50 a year more than I would like to spend to be able to do what I want.
Hamachi also has a UI that I simply don't really like the look of, and you HAVE to keep it open, or else it will shut down.
I love using Zerotier.
It very nearly never gives us problems, it has no UI to keep open, it has a ridiculous amount of customization, and it is extremely easy to setup for a user.
Every time I help someone set it up, it takes at most about 5 minutes, and that is for people that know absolutely nothing about computers.
Zerotier supports 25 devices for free, then also significantly ramps up to $5 a month ($60 a year) per 25 users more ($2.40 per user).
While the pricing here is also bad at more than 25 devices, it supports 20 more devices than Hamachi for free.
If you look at the Zerotier pricing page, however, there is a free plan that has unlimited users.
That is an open source network, where you host your Zerotier network, give it a bunch of servers, and in turn just use the Zerotier command line.
If you read the 4th sentence, and didn't immediately understand why you would want to use a P2P VPN, then this is not for you.
The amount of times my group has wanted something like this, for stuff like Minecraft private servers, locally hosted Teamspeak servers, etc. is honestly too many to count.
As my group is fairly small (about 5 people + 1 Laptop + 2 Servers and 1 Phone), I would rather use Zerotier instead of paying $50 a year.