Stack Overflow
Q&A for professional and enthusiast programmers
Cross Validated
Q&A for statisticians, data analysts, data miners and data visualization experts
Ask Different
Q&A for power users of Apple hardware and software
Meta Stack Overflow
Q&A for meta-discussion of the Stack Exchange family of Q&A websites
Bicycles
Q&A for people who build and repair bicycles, people who train cycling, or commute on bicycles
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.
Server Fault
Q&A for professional system and network administrators
Geographic Information Systems
Q&A for cartographers, geographers and GIS professionals
Database Administrators
Q&A for database professionals who wish to improve their database skills and learn from others in the community
Biology
Q&A for biology researchers, academics, and students
Raspberry Pi
Q&A for users and developers of hardware and software for Raspberry Pi
Gardening & Landscaping
Q&A for gardeners and landscapers