So I'm a bit of a noob at this Arduino stuff, but then thats allowed considering I've only had it about 4 days, right? 
Anyway, this is the coolest thing I've built yet. It's a little keyboard that pumps out data which is then converted to midi in Processing, which then gets sent to Ableton Live. I think the coolest thing about it is that it would be SUPER easy to extend the number of buttons on this, as to add two extra buttons requires the addition of only one extra output pin! If it wasn't for my tiny breadboard (and lack of buttons...) I'd have made it have a full octave!
Anyway, here's a little video:
And the schematic from which I copied the button reading method:
(The bit at the bottom left)
I had to modify it slightly to work with the Arduino, in the form of some pulldown resistors on the inputs and replacing the diodes with LEDS (and, of course, resistors).
To read the inputs, I do it in stages. From that schematic (ignoring the higher 4 buttons), first I drive pin 10 high and pin 13 low, then read the state of pin 14 and 15. Then I drive pin 10 low and pin 13 high, and read pin 14 and 15. Easy!
I think this is a pretty nice way of reading buttons. Not very useful for 4 or fewer buttons, but definitely a nice way of saving some pins if you have more!
Cheers for reading, let me know your thoughts and of any improvements you would make! 
