Stack Overflow
Q&A for professional and enthusiast programmers
Ask Different
Q&A for power users of Apple hardware and software
Area 51
Stack Exchange Network staging zone, where users come together to build new Q&A sites
Server Fault
Q&A for professional system and network administrators
Physics
Q&A for active researchers, academics and students of physics
Mathematics
Q&A for people studying math at any level and professionals in related fields
Programmers
Q&A for professional programmers interested in conceptual questions about software development
Meta Stack Overflow
Q&A for meta-discussion of the Stack Exchange family of Q&A websites
Photography
Q&A for professional, enthusiast and amateur photographers
Ask Patents
Q&A for people interested in improving and participating in the patent system
English Language Learners
Q&A for speakers of other languages learning English
Russian Language & Usage
Q&A for students, teachers, and linguists wanting to discuss the finer points of the Russian language
