I probably spend more time than I should in SO.
Q&A for professional and enthusiast programmers
Q&A for users of TeX, LaTeX, ConTeXt, and related typesetting systems
Q&A for people studying math at any level and professionals in related fields
Q&A for users of Linux, FreeBSD and other Un*x-like operating systems.
Q&A for cartographers, geographers and GIS professionals
Q&A for meta-discussion of the Stack Exchange family of Q&A websites
Q&A for Ubuntu users and developers
Q&A for people interested in statistics, machine learning, data analysis, data mining, and data visualization