RiyazStudio combines tanpura and tabla machines in a responsive and easy-to-use PC-based practice environment for North Indian classical music. RiyazStudio uses real tabla loops - 150 of them - in contrast to often mechanical sounding tabla machines that set individual tabla bols at rigidly set intervals. Whatever pitch you sing or play at you'll have a choice of different tabla tunings in the main talas. You can even adjust the tabla style without changing speed.Overall I like this software and am planning to buy it. It costs close to $40. You can get a free temporary license and use it for 30 days before actually deciding to buying it. Some of the features would be disabled in the free temporary license. And if you actually buy it during the trial period of 30 days, you get a $5 discount. I don't sing, but I am planning to use it in some of my compositions.
According to their FAQ, they seem to support only Windows PC. So if you have a Mac, I am not sure if their suggested workaround would work.
Update (Sep 11, 2008): It seems if we want to use these samples in our recordings, then the license doesn't permit it. Here is an extract from their license agreement:
This license agreement grants permission to use the supplied sound samples for practice purposes. Distribution or broadcast of these sound samples or recordings derived from them is not granted by this license; contact us at firstname.lastname@example.org if you wish do this.