This site presents an interesting logic calculator:
http://www.inf.unibz.it/~franconi/teaching/propcalc/
These sites considers logic gates (with links to other logic topics):
http://www.jhu.edu/~virtlab/logic/log_cir.htm
This site, Basic Gates and Functions provides a nice discussion of logic circuits:
http://www.ee.surrey.ac.uk/Projects/Labview/gatesfunc/index.html