Modal Logic
- Open Encyclopedia article.
- http://open-site.org/Science/Mathematics/Logic/Modal_Logic/

Modal epistemic logic
- An extensive overview of the subject.
- http://stinfwww.informatik.uni-leipzig.de/~duc/Thesis/node8.html

Mally's Deontic Logic
- Discussion of Ernst Mally's logic of obligation; from the Stanford Encyclopedia by Gert-Jan Lokhorst.
- http://plato.stanford.edu/entries/mally-deontic/

Modality, Si! Modal Logic, No!
- Article by J. McCarthy on the use of modality in artificial intelligence.
- http://www-formal.stanford.edu/jmc/modality/modality.html

The Calculus of Structures - Modal Logics
- Several normal propositional modal logics are systematically presented in the calculus of structures and cut elimination is proved. By Alessio Guglielmi.
- http://alessio.guglielmi.name/res/cos/ML/

Modal Logics
- A concise introduction to modal logics by Anthony A. Aaby.
- http://cs.wwc.edu/~aabyan/Logic/Modal.html

Computational Tools for Modal Logic
- Resources collected by Renate Schmidt.
- http://www.cs.man.ac.uk/~schmidt/tools/