Kommandr is described as 'had a simple yet ambitious charter: be a social application that enables users to discover, learn about, save, and share commands. Kommandr sees to the first two charter items' and is an website in the OS & Utilities category. There are four alternatives to Kommandr, not only websites but also apps for Linux, Mac and Windows. The best alternative is TLDR pages, which is both free and Open Source. Other great sites and apps similar to Kommandr are cheat (Free, Open Source), Linux man-pages (Free, Open Source) and bropages (Free, Open Source).
Cheat allows you to create and view interactive cheatsheets on the command-line. It was designed to help remind *nix system administrators of options for commands that they use frequently, but not frequently enough to remember.