GitHub Repository Size - Browser extension to show file and repo size in GitHub
LaunchKit - Open Source - A popular suite of developer tools, now 100% open source.
GitNews App - Trending repositories from GitHub, HackerNews & Reddit
SmallDevTools - Handy developer tools with a delightful interface
Roundup - Email digest of new releases from your starred GitHub repos
GitHub Student Developer Pack - The best developer tools, free for students.