GitHub Visualizer is described as 'Visualize history of repository changes for GitHub repos'. There are six alternatives to GitHub Visualizer for Online / Web-based, Self-Hosted solutions, Mac, Windows and Linux. The best alternative is Gource, which is both free and Open Source. Other great apps like GitHub Visualizer are CodeFlower (Free, Open Source), Git Visualizer (Free, Open Source), Codeology (Free, Open Source) and Repography (Freemium).
Software projects are displayed by Gource as an animated tree with the root directory of the project at its centre. Directories appear as branches with files as leaves. Developers can be seen working on the tree at the times they contributed to the project.