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English Language & Usage
Q&A for linguists, etymologists, and serious English language enthusiasts
Stack Overflow
Q&A for professional and enthusiast programmers
Meta Stack Overflow
Q&A for meta-discussion of the Stack Exchange family of Q&A websites
Programmers
Q&A for professional programmers interested in conceptual questions about software development
Mathematics
Q&A for people studying math at any level and professionals in related fields
Area 51
Stack Exchange Network staging zone, where users come together to build new Q&A sites
TeX - LaTeX
Q&A for users of TeX, LaTeX, ConTeXt, and related typesetting systems
Server Fault
Q&A for professional system and network administrators
Photography
Q&A for professional, enthusiast and amateur photographers
Game Development
Q&A for professional and independent game developers
Cross Validated
Q&A for statisticians, data analysts, data miners and data visualization experts
Stack Apps
Q&A for apps, scripts, and development with the Stack Exchange API
Physics
Q&A for active researchers, academics and students of physics
Android Enthusiasts
Q&A for enthusiasts and power users of the Android operating system
Theoretical Computer Science
Q&A for theoretical computer scientists and researchers in related fields
Programming Puzzles & Code Golf
Q&A for programming puzzle enthusiasts and code golfers
Writers
Q&A for authors, editors, reviewers, professional writers, and aspiring writers
Answers OnStartups
Q&A for entrepreneurs looking to start or run a new business
Audio-Video Production
Q&A for engineers, producers, editors, and enthusiasts spanning the fields of audio, video, and media creation