VulnHub is a website offering a wide variety of “Boot2Root” vulnerable virtual machines available for visitors to download free-of-charge. It predates HackTheBox by over a decade, with its first VM, BadStore: 1.2.3, having been posted in 2004.
Most of the VMs on VulnHub are designed to be compatible with either VMWare or VirtualBox. If you’d like to follow along on your own system, you’ll need one of the two installed.
Also, it is advised to use Host-Only Networking to communicate between your Kali VM and the various VulnHub targets. This allows you to interact with the target as if they’re on a local network, while keeping them segregated from the Internet as a whole.