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MIT OpenCourseWare (OCW) provides free, openly licensed course materials from MIT across many subjects. It offers math courses (including set theory) with lecture notes, assignments, exams, and sometimes video lectures for self-guided study.
More resources on Set Theory
ProofWiki - Set Theory
Wiki with proofs and definitions
Stanford Encyclopedia of Philosophy - Set Theory
In-depth free encyclopedia entry
Discrete Mathematics
Discrete mathematics forms the mathematical foundation of computer and information science. It is also a fascinating subject in itself. Learners will become familiar with a broad range of mathematical objects like sets, functions, relations, graphs, that are omnipresent in computer science. Perhaps more importantly, they will reach a certain level of mathematical maturity - being able to understand formal statements and their proofs; coming up with rigorous proofs themselves; and coming up with interesting results. This course attempts to be rigorous without being overly formal. This means, for every concept we introduce we will show at least one interesting and non-trivial result and give a full proof. However, we will do so without too much formal notation, employing examples and figures whenever possible. The main topics of this course are (1) sets, functions, relations, (2) enumerative combinatorics, (3) graph theory, (4) network flow and matchings. It does not cover modular arithmetic, algebra, and logic, since these topics have a slightly different flavor and because there are already several courses on Coursera specifically on these topics.
Mathematics for Computer Science (6.042J)
Master set theory and other math essentials for computer science with this comprehensive course from Meyer & Rubinfeld.
mathworld.wolfram.com
MathWorld is Wolfram’s comprehensive online mathematics encyclopedia. It offers detailed articles on topics across math—including set theory—featuring definitions, theorems, proofs, examples, and diagrams with links to related concepts.
Naive Set Theory
Naive Set Theory - A book resource