In the late 1990's, a product called MixMan allowed you to create your own remixes of popular artists loops on your PC. Now, there is an equally sophisticated product called BeatMaker for your iPhone. The following YouTube video gives you a sense of the app's sophistication..
Although the price ($19.99) of the BeatMaker iPhone app may seem steep -- make no mistake, this is a full software program, with a serious depth and range. If you like playing around with beats, BeatMaker is the tool for you