English Language & Usage
Q&A for linguists, etymologists, and serious English language enthusiasts
Biology
Q&A for biology researchers, academics, and students
Unix & Linux
Q&A for users of Linux, FreeBSD and other Un*x-like operating systems.
Android Enthusiasts
Q&A for enthusiasts and power users of the Android operating system
Area 51
Stack Exchange Network staging zone, where users come together to build new Q&A sites
Photography
Q&A for professional, enthusiast and amateur photographers
Physical Fitness
Q&A for physical fitness professionals, athletes, trainers, and those providing health-related needs
Bicycles
Q&A for people who build and repair bicycles, people who train cycling, or commute on bicycles
Programmers
Q&A for professional programmers interested in conceptual questions about software development
Physics
Q&A for active researchers, academics and students of physics
Server Fault
Q&A for professional system and network administrators
User Experience
Q&A for user experience researchers and experts
Science Fiction & Fantasy
Q&A for science fiction and fantasy enthusiasts
TeX - LaTeX
Q&A for users of TeX, LaTeX, ConTeXt, and related typesetting systems
Mathematics
Q&A for people studying math at any level and professionals in related fields
Academia
Q&A for academics and those enrolled in higher education
Cognitive Sciences
Q&A for practitioners, researchers, and students in cognitive science, psychology, neuroscience, and psychiatry
Computational Science
Q&A for scientists using computers to solve scientific problems
Quantitative Finance
Q&A for finance professionals and academics
Software Quality Assurance & Testing
Q&A for software quality control experts, automation engineers, and software testers
Meta Stack Overflow
Q&A for meta-discussion of the Stack Exchange family of Q&A websites
Answers OnStartups
Q&A for entrepreneurs looking to start or run a new business
Cross Validated
Q&A for statisticians, data analysts, data miners and data visualization experts