Mathematics
Q&A for people studying math at any level and professionals in related fields
Unix & Linux
Q&A for users of Linux, FreeBSD and other Un*x-like operating systems.
Area 51
Stack Exchange Network staging zone, where users come together to build new Q&A sites
Programmers
Q&A for professional programmers interested in conceptual questions about software development
Cryptography
Q&A for software developers, mathematicians and others interested in cryptography
Computer Science
Q&A for students, researchers and practitioners of computer science
Theoretical Computer Science
Q&A for theoretical computer scientists and researchers in related fields
Web Applications
Q&A for power users of web applications
Mi Yodeya
Q&A for those who base their lives on Jewish law and tradition and anyone interested in learning more
Photography
Q&A for professional, enthusiast and amateur photographers
Meta Stack Overflow
Q&A for meta-discussion of the Stack Exchange family of Q&A websites
Ask Different
Q&A for power users of Apple hardware and software
English Language & Usage
Q&A for linguists, etymologists, and serious English language enthusiasts
Server Fault
Q&A for professional system and network administrators
TeX - LaTeX
Q&A for users of TeX, LaTeX, ConTeXt, and related typesetting systems
Cognitive Sciences
Q&A for practitioners, researchers, and students in cognitive science, psychology, neuroscience, and psychiatry
Personal Productivity
Q&A for people wanting to improve their personal productivity
Programming Puzzles & Code Golf
Q&A for programming puzzle enthusiasts and code golfers
Software Quality Assurance & Testing
Q&A for software quality control experts, automation engineers, and software testers
Computational Science
Q&A for scientists using computers to solve scientific problems
The Workplace
Q&A for members of the workforce navigating the professional setting