Wippien

Wippien is a very outdated software, but still is one of the best alternatives out there to compete with Hamachi and such. The UI is simple and friendly. Wippien establishes direct connection between peers by penetrating NAT; there is no relayed server, so it might (very rarely) not work for certain people. The main downside of Wippien is that it doesn't set up the firewall properly, rendering it unusable. You'll have to create an outbound firewall rule for Wippien.exe file allowing any connection, literally. On inbound side you must edit one of the Wippiens' rules (there are 2) to allow any connection for Wippien.exe, just like as the outbound rule. Then edit the other inbound rule to allow any protocols and programs, on scope tab you must add the address 5.0.0.0/8 in both empty boxes, on advanced tab you must allow only public networks, click on customize and allow only local area network. Another workaround is to disable firewall completely, but that obviously will become a security flaw.

NOTE: The firewall setup is not required anymore (unless you want to check your peers' ping; check the second note)! I contacted the main developer of Wippien, reported the issue. They fixed it within 2 days or so. NOTE 2: Realize that pinging peers on Wippien doesn't always works. This doesn't necessarily means there is something wrong with Wippien, nor the connection. If the IP of your contacts is black, the direct connection is already set and established.