The CodeHelp page, a program to aid in the decryption (and encryption) of messages using "paper-and-pencil" methods found in the book Elementary Cryptanalysis by Abraham Sinkov, revised and updated by Todd Feil, and published by the MAA in 2009. You can download the errata.
You can download errata (a pdf file) for A First Course in Abstract Algebra: Rings, Groups, and Fields, 2nd edition, published by Chapman & Hall/CRC Press, 2005, by Anderson and Feil. (Last update: 15 December, 2008.)
You can download errata (a pdf file) for Essential Discrete Mathematics for Computer Science (Prentice-Hall) by Feil and Krone. (Last update: 15 December 2008.)
Here is an origami folding pattern for a Denison Block D that you can make with one straight cut.
Here are Pringle can Enigma cutouts, with instructions. These cutouts come in four flavors: regular sized can, regular sized can pre-wired, tall can, and tall can pre-wired. (Thanks to Stuart Boersma from Central Washington University for the rotor wiring on the regular sized can.) Be sure to print these as actual size, not scaled to fit page. This idea seems to originate with the people doing the National Cipher Challenge in the UK, but the templates are no longer available. I made up my own here.
I'm past director of the Denison Spring Programming Contest, held the last Saturday of each February.
Here's a link to some things I'm interested in.