Stack Overflow
Q&A for professional and enthusiast programmers
Area 51
Stack Exchange Network staging zone, where users come together to build new Q&A sites
Meta Stack Overflow
Q&A for meta-discussion of the Stack Exchange family of Q&A websites
Server Fault
Q&A for professional system and network administrators
Geographic Information Systems
Q&A for cartographers, geographers and GIS professionals
Graphic Design
Q&A for professional graphic designers and non-designers trying to do their own graphic design
Bicycles
Q&A for people who build and repair bicycles, people who train cycling, or commute on bicycles
User Experience
Q&A for user experience researchers and experts
Mathematics
Q&A for people studying math at any level and professionals in related fields
Theoretical Computer Science
Q&A for theoretical computer scientists and researchers in related fields
Programmers
Q&A for professional programmers interested in conceptual questions about software development
Cross Validated
Q&A for statisticians, data analysts, data miners and data visualization experts
English Language & Usage
Q&A for linguists, etymologists, and serious English language enthusiasts
Cryptography
Q&A for software developers, mathematicians and others interested in cryptography
Answers OnStartups
Q&A for entrepreneurs looking to start or run a new business