Interested in: most kinds of mechanics, history of engineering, geometric methods in physical sciences, pedagogy, history, poetry.
Mathematics
Q&A for people studying math at any level and professionals in related fields
Area 51
Stack Exchange Network staging zone, where users come together to build new Q&A sites
Musical Practice & Performance
Q&A for musicians, students, and enthusiasts
Physics
Q&A for active researchers, academics and students of physics
Biology
Q&A for biology researchers, academics, and students
Stack Overflow
Q&A for professional and enthusiast programmers
Writers
Q&A for authors, editors, reviewers, professional writers, and aspiring writers
TeX - LaTeX
Q&A for users of TeX, LaTeX, ConTeXt, and related typesetting systems
English Language & Usage
Q&A for linguists, etymologists, and serious English language enthusiasts
Meta Stack Overflow
Q&A for meta-discussion of the Stack Exchange family of Q&A websites
Photography
Q&A for professional, enthusiast and amateur photographers
Cross Validated
Q&A for statisticians, data analysts, data miners and data visualization experts