ncatlab.org
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nLab is a collaborative online encyclopedia of higher mathematics, especially category theory, higher algebra, topology, and related fields, with wiki-style articles, definitions, and expository notes by researchers. It provides in-depth entries on fundamental concepts such as the fundamental group and fundamental groupoid, along with discussions of higher categories, homotopy theory, and links to literature.
More resources on Fundamental Group
nLab: Fundamental Group
Comprehensive reference with resources and links
Hatcher's Algebraic Topology
Free online book with chapter on fundamental groups
Introduction to Algebraic Topology
Learn algebraic topology with N J Wildberger! This course explores the fundamental group using a novel approach.
mathworld.wolfram.com
MathWorld is Wolfram's comprehensive online mathematics encyclopedia. It offers concise, authoritative articles on topics (including the fundamental group) with definitions, theorems, proofs, examples, diagrams, and curated references.
Topology
Topology - A book resource
ocw.mit.edu
MIT OpenCourseWare (OCW) is a free, openly accessible collection of MIT course materials spanning thousands of courses. It includes lecture notes, problem sets, exams, and video lectures for self-study, including topics like the Fundamental Group in mathematics.