The link:
Goranr:
http://www.instructables.com/id/Arduino-3-wire-Matrix-Keypad/
is very similar to:
http://www.avr-asm-tutorial.net/avr_en/keypad/keyboard.html#adc
The second link also contains a spreadsheet for calculating resistor values. I modified the spreadsheet to calculate values for a 4x4 matrix.
The problem I have with this solution is that for the 3x4 matrix it uses 7 resistors of 6 different values. I would like to find a solution that uses 10 or 12 resistor of all the same value. Or maybe 5 or 6 resistors of one value and 5 or 6 of another value.
The reason I am thinking this might be a better solution is that while the component count is higher, the cost of resistors goes down with volume. Rather than buy 6 packages of 5 to 10 resistors each, it would be cheaper to by 1 package of 100 resistors.
Also, the tolerances are important. If I buy a 100 resistors they they were probably made at around the same time with the same material and if there is an error they are probably all off by about the same amount in the same direction. So if they are all low by 5% then the errors will probably cancel in a design that uses all the same value. Rather than 6-8 different values with some high and some low. Since I don't have a design, this is all assumption.
There may also be an inexpensive SIP or DIP package out there that contains the appropriate resistor network. I just haven't come across it.