Nonstandard Logics and Extensions 

A comprehensive listing of flavours of nonstandard logic with brief descriptions and references, compiled by Peter Suber.
Article in the Stanford Encyclopaedia of Philosophy by John L. Bell. Infinitary Logic is a branch of formal logic where finitary formulae are replaced by potentially infinitary mathematical entities.
Shows how a number of representative logics fit together. The interrelationships usually given as something of the form: System X is system Y plus the axiom Z. By John Halleck.
Notes defining the subject. Available in HTML and PS formats.
Wikipedia (free encyclopedia) article.
