Extending Set Theory
- http://web.mit.edu/dmytro/www/NewSetTheory.htm
- 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.
New Foundations
- http://math.boisestate.edu/~holmes/holmes/nf.html
- 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.