For the speaker I used pretty much the exact circuit used by BrunoIP in his instructable here. The only change is that I used the 5V supply from the Raspberry PI. R1 is 4.7KOhm, R2 is 1M. Q1 can be a 2N3904 (NPN) and Q2 can be 2N3906 (PNP). C1 is a 10uF electrolytic capacitor. Oh and I threw in the obligatory power LED with a 1K resistor between +5 and the anode.

As you can see from the photo of my project I just threw it together on a mini breadboard and put it in a high tech enclosure (tupperware with a hole cut in the lid into which I hot glued the speaker).