A Shruti Box is a classical Indian musical instrument which sustains a constant drone of a combination of user-set notes. In other words, you select which pitches you want in your drone, then you start pumping the Shruti and it sustains a constant drone at the specified intervals. It's a lot like a Harmonium, except that instead of keys it simply has little covers over each of the reed holes. You open the holes corresponding the pitches you want played.
The Shruti is useful for many different applications. They are useful in performance, especially because they lay down a kind of carpet on which a singer or instrumentalist can lay down something special. They are also useful for practice since you can, for instance, set the Shruti to drone the home-tone of a chant you are playing with. As you improvise and play around you can simultaneously hear how you sound against your musical home base.
I was was first introduced to the Shruti by the St. Gregory of Nyssa folks. They love a good drone! It's easy to get such a sound out of a congregation, but you can also simulate a Shruti with a pipe organ pretty easily (you just need something or someone to hold the right keys down).
Of course, there is also an iPhone app for that. And they sound surprisingly good. You just need to have a sound system to hook up to your iPhone (or iPod Touch) and you've got yourself an instant Shruti box! Here is an example of what the Shruti sounds like. This musician will play over the top of it with a Hang Drum, which is a whole post in its own right!