GitHub + Slack Integration - Full visibility into your GitHub projects right inside Slack
Refined GitHub - Browser extension that makes GitHub cleaner & more powerful
GitCommands - An interactive reference tool for Git commands
GitZip - Download or create a download link for a GitHub project folder/sub-folder or file.
ask-stack - Get answers from Stack Overflow directly in the Atom editor
GitHub Hovercard - GitHub Hovercard provides neat hovercards for GitHub.