The Switchboard Module uses 2x 4021 Shift Registers to read 16 buttons simultaneously. This is the same circuit as 2 NES controllers, so it only requires 4 pins, and you read the status of the buttons the same way. It fits on the Propeller Platform, or on any protoboard or breadboard.
I put it together because I wanted to do a soundboard (where you push a button and it plays back an audio sample), but I thought buttons would be more ideal than an NES controller. Of course, it works well for any project where you want a lot of buttons but don't want to use too many pins.
You can learn more or pick one up on the project page. The schematic is also up there along with an object for using it with the Prop. Everything's available under the MIT license, you can download the diptrace layout off the project page, too.
Let me know if you have any questions!