Mathematics
Q&A for people studying math at any level and professionals in related fields
Puzzling
Q&A for those who create, solve, and study puzzles
Stack Overflow
Q&A for professional and enthusiast programmers
Code Golf
Q&A for programming puzzle enthusiasts and code golfers
Code Review
Q&A for peer programmer code reviews
Computer Science
Q&A for students, researchers and practitioners of computer science
Arqade
Q&A for passionate videogamers on all platforms
Super User
Q&A for computer enthusiasts and power users
Unix & Linux
Q&A for users of Linux, FreeBSD and other Un*x-like operating systems
MathOverflow
Q&A for professional mathematicians
Worldbuilding
Q&A for writers/artists using science, geography and culture to construct imaginary worlds and settings
Motor Vehicle Maintenance & Repair
Q&A for mechanics and DIY enthusiast owners of cars, trucks, and motorcycles
Politics
Q&A for people interested in governments, policies, and political processes