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Unix & Linux
Q&A for users of Linux, FreeBSD and other Un*x-like operating systems.
Server Fault
Q&A for professional system and network administrators
Area 51
Stack Exchange Network staging zone, where users come together to build new Q&A sites
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
Game Development
Q&A for professional and independent game developers
Drupal Answers
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Programmers
Q&A for professional programmers interested in conceptual questions about software development
Photography
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Mathematics
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Meta Stack Overflow
Q&A for meta-discussion of the Stack Exchange family of Q&A websites
Musical Practice & Performance
Q&A for musicians, students, and enthusiasts
Audio-Video Production
Q&A for engineers, producers, editors, and enthusiasts spanning the fields of audio, video, and media creation
Answers OnStartups
Q&A for entrepreneurs looking to start or run a new business
Ask Different
Q&A for power users of Apple hardware and software