Shellter is described as 'Labs is an online platform that aims to help you improve your cyber security skills through training materials and practical challenges'. There are three alternatives to Shellter, not only websites but also apps for a variety of platforms, including VirtualBox, VMware Fusion, VMware Player and VMware Workstation. The best alternative is Hack The Box, which is free. Other great sites and apps similar to Shellter are VulnHub (Free) and HackThisSite (Free, Open Source).
Hack The Box is an online platform allowing you to test your penetration testing skills and exchange ideas and methodologies with other members of similar interests. It contains several challenges that are constantly updated.