Stack Overflow
Q&A for professional and enthusiast programmers
TeX - LaTeX
Q&A for users of TeX, LaTeX, ConTeXt, and related typesetting systems
Unix & Linux
Q&A for users of Linux, FreeBSD and other Un*x-like operating systems.
Theoretical Computer Science
Q&A for theoretical computer scientists and researchers in related fields
Mathematics
Q&A for people studying math at any level and professionals in related fields
Server Fault
Q&A for professional system and network administrators
Cryptography
Q&A for software developers, mathematicians and others interested in cryptography
English Language & Usage
Q&A for linguists, etymologists, and serious English language enthusiasts
Database Administrators
Q&A for database professionals who wish to improve their database skills and learn from others in the community
Meta Stack Overflow
Q&A for meta-discussion of the Stack Exchange family of Q&A websites
Quantitative Finance
Q&A for finance professionals and academics
Linguistics
Q&A for professional linguists and others with an interest in linguistic research and theory