Bounded Set Theory - A weak version of ordinary set theory using bounded quantification. Papers and software.
Cantor's Theorem - Article in the Platonic Realms, describing Cantor's diagonal argument that showed that 'infinite integers' can be ordered.
Consequences of the Axiom of Choice Project - Project to keep the book (also named in the title), describing forms related to the Axiom of Choice and their implications, updated.
Equivalence Relation - Encyclopedia entry on equivalence relations and their role in set theory.
Extending Set Theory - Extends the language of set theory through restricted self-reference and through certain large cardinals. Also discusses higher order set theory and axiomatization through reflection principles.
Metamath Proof Explorer - Over 3000 complete formal proofs deriving numbers and beyond from Zermelo-Fraenkel set theory axioms.
New Foundations - Set theory introduced by W. V. O. Quine in 1937. This is a refinement of Russell's theory of types based on the observation that the types in Russell's theory look the same, as far as one can apparently prove.
A New System of Axioms Instead of ZF - The page claims that "the axioms shown below have the contents that should overturn the set theory of today".
Programming with Sets - Using set-theoretic primitives as a conceptual tool in programming, includes discussion of SETL and MIRANDA languages.
Set Theoretic Analysis. - On a part of math where Set Theory, Topology and Analysis meet. Has surveys, preprints, conference announcements, book reviews and problems.
Set Theorists - A list of email addresses and affiliations.
Set Theory - Directory of set theorists, maintained by Jean A. Larson.
Set Theory - Survey from the Stanford Encyclopedia of Philosophy by Thomas Jech.
Set Theory - From Dave Rusin's "Known Math" collection.