The iPhone App Store offers several zen garden applications and today, I was able to play with one of them, iZen Garden [iTunes link], on my simulator. The program allows you to place stones onto a bed of sand and rake the sand around the stones. As the marketing materials suggest, it's basically a fidget toy. "iZen Garden is perfect for long meetings, train rides, or meditation practice. It helps you to center your mind, relax your psyche, and relieve your stress."
The interface is easy enough to use, with several rocks available and reset options for when your perfect garden gets too messy. At $4.99, it's a love it or leave it program. If rock placement and sand stroking isn't your thing, then iZen Garden probably isn't either. If it is, this is a simple visually appealing solution.
I found the program easy-to-use and modestly fun. I have limited artistic skills and my raked gardens always looked messy. Your mileage will almost certainly vary.
iZen Garden works on both iPhone and iPod touch.