You can use a single analog pin to read an entire keypad.
First, read up on voltage dividers - they let you convert a resistance into a voltage. You will need to know this later on.
A keypad typically has 4 inputs, and 3 outputs (or vice-versa). Label the 4 pins A,B,C,D and the 3 pins E, F, G. You will be wiring A up to a voltage supply, and G up to an analog pin, and via a suitable resistor to ground. This becomes your voltage divider.
Then, put a resistor between A+B, B+C, C+D, and between E+F, F+G. So, if you press the button that connects A to G, there will be no resistance (=+5V). If you press the button that connects A to F, then the resistance will be that of the resistance F->G. If you press the button that connect D and E, the circuit goes: VCC-A-resistor-B-resistor-C-resistor-D-pushbutton-E-resistor-F-resistor-G - Arduino analog pin. With no button pressed, the voltage will be 0.
You will need to work out different resistors in advance, to give you as even a range as possible between 0 and 5 volts (the critical thing being the smallest difference). You can calculate the resistance of any combination here: http://www.calculator.net/resistor-calculator.html and you can use Ohm's law to calculate the final voltage. This depends on what you have in your kit. I ended up making a spreadsheet, and then trying random resistor values until I got a good spread.
DO NOT use any resistors under 1kOhm.
Once you have the right spread, make it up on a solderless breadboard. Measure the voltage with your arduino (use the "readanalogvoltage" sketch, take out
* (5.0 / 1023.0) , and make up a table - press button 1, you get a reading if A, press button 2, get a reading of B. Include the reading when no button is pressed, which should be 0.
Then find the numbers exactly in between.
For example, if the lowest three readings are 0,20,37 for no button, A, and B pressed, respectively, then in your code, you will say: if the pin measures less than (half way between 0 and 20=) 10, then no button is pressed; else if the pin measures less than (half way between 20 and 37=) 29, then button A is pressed... and so forth.