Beginner
Try Git (try.github.io)
The Simple Guide (rogerdudler.github.io)
Git Immersion (gitimmersion.com)
Getting Started (git-scm.com)
Git Basics (www.atlassian.com)
Git From the Inside Out (codewords.recurse.com)
What is Version Control? (git-scm.com)
Working with Branches (www.youtube.com)
Version Control Best Practices (www.git-tower.com)
10 Common Git Problems and How to Fix Them (www.codementor.io)
GitHub Integration (devcenter.heroku.com)
Oh Shit, Git! (ohshitgit.com)
Intermediate
Git Best Practices (sethrobertson.github.io)
Learn Git Branching (pcottle.github.io)
Collaborating (www.atlassian.com)
Migrating to Git (www.atlassian.com)
Advanced
Git from the bottom up (ftp.newartisans.com)
A Hacker's Guide to Git (wildlyinaccurate.com)
Advanced Tips (www.atlassian.com)