ncatlab.org
Unknown
nLab is a collaborative online encyclopedia for advanced mathematics and theoretical computer science, with a core emphasis on category theory, higher categories, and homotopy theory. It provides rigorously defined concepts, explanations, diagrams, and extensive cross-references, covering Lie groups, Lie algebras, and other Lie-theoretic topics as part of its broad scope.
More resources on Lie Groups
mathworld.wolfram.com
MathWorld is Wolfram's comprehensive online mathematics encyclopedia, with detailed articles, definitions, formulas, and visualizations across a wide range of topics. It includes entries on Lie groups, providing explanations, formulas, examples, and references.
ocw.mit.edu
MIT OpenCourseWare offers free, openly licensed course materials from MIT across many subjects, including mathematics. You can access lecture notes, problem sets, exams, and video lectures from MIT courses, which can support learning topics like Lie groups.