Thanks to a Twitter post from @SmartMusic I have found a neat little online rhythm generator that can easily be used to provide background accompaniments when having students practice scales and other exercises. Having the background beat going really does help younger students keep focused and steady, plus it makes even simple sounding exercises a lot more impressive to the ear. Best of all it's free and fairly easy to use.
What JamStudio reminds me of is a stripped down remake of Band In A Box. You set up the chord changes in chunks of up to 16 bars, select the instruments and styles of your synthesized band, and hit play. The song can be set up to loop over and over or set to play through once and end. You can set the accompaniment to be either 4 or 3 beats per bar and can adjust the tempo as well.
You can do a lot here without even logging in. Creating an account opens up additional options like saving your songs and giving you the chance to pay for the All Access Pass which grants you the rights to download the audio of your songs for use however you see fit.
All in all JamStudio is a nice little online app that does fill a need if you are not in the market for buying a commercial accompaniment product with similar functions.