Stack Overflow
Q&A for professional and enthusiast programmers
Programmers
Q&A for professional programmers interested in conceptual questions about software development
Server Fault
Q&A for professional system and network administrators
Android Enthusiasts
Q&A for enthusiasts and power users of the Android operating system
Physics
Q&A for active researchers, academics and students of physics
User Experience
Q&A for user experience researchers and experts
Web Applications
Q&A for power users of web applications
Database Administrators
Q&A for database professionals who wish to improve their database skills and learn from others in the community
Mathematics
Q&A for people studying math at any level and professionals in related fields
Stack Apps
Q&A for apps, scripts, and development with the Stack Exchange API
Unix & Linux
Q&A for users of Linux, FreeBSD and other Un*x-like operating systems.
English Language & Usage
Q&A for linguists, etymologists, and serious English language enthusiasts
Electrical Engineering
Q&A for electronics and electrical engineering professionals, students, and enthusiasts
Theoretical Computer Science
Q&A for theoretical computer scientists and researchers in related fields
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
Area 51
Stack Exchange Network staging zone, where users come together to build new Q&A sites