If you're considering adding keypad-access control to an electric door-strike or any other security application, check out the "keyentry" project by Arduino enthusiast Dean Fogarty. It is simple to follow, and easy to replicate yourself with any typical keypad available from electronics retailers here and abroad. The project was designed to control two strikes, however you can easily add more using the relay control boards shown below.
If you're looking to control relays and other higher-current devices - it's much easier to use dedicated relay control boards. Here at Freetronics we have the RELAY4: board, perfect for controlling up to four relays with Arduino (or any other microcontroller's) digital output pins: