The CUI GF0661 8 ohm, .5 watt speaker fits perfectly in a low-cost, readily accessible enclosure, (a styrofoam cup), and has a range of 120hz-10KHz. If you don’t know the difference an exposure makes in a small speaker, try just cupping your hands behind one, the next time you have audio playing in a project. The difference is quite dramatic. With or without an enclosure, this speaker is loud enough with just an Arduino pin, without further amplification, to make some great audio projects.
It’s fun to experiment with Arduino music. Here’s a few links including, the Arduino tone tutorial, and the RTTTL demonstration for playing ringtones on your Arduino (you’ll need the Arduino Tone Library.)
Here’s the frequency response curve of the speaker. Its a little irregular, but its not really significant. The response of popular piezo speakers looks (and sounds) much worse, and the enclosure improves the curve dramatically. You can also hide light sensors and motion detectors behind the speaker. Plus, the speaker looks great, which makes your project one step classier.