People want digital statements like "you're safe if the voltage is less than X volts". Unfortunately, the real world doesn't work that way. You can quote industrial or government standards that state that a particular voltage level is safe, but I can find cases where people were injured with lower voltage levels. My canonical example is a person working on the 12 volt system of a car. Everyone thinks this is safe -- until I give them the example of a relative who was working on a car and got his metal ring caught between a + battery terminal and ground -- and got a hell of a burn.
So the evaluation of risk also involves knowing how much energy the circuit can deliver, regardless of the voltage.
I like ronsimpson's statement: "30V AC or DC is pretty safe. I would keep my hands dry!" He's right on -- but you could still be injured if e.g. a sharp edge of a conductor penetrated the skin and greatly reduced the resistance. Many people poo-poo these corner situations -- except perhaps the person who is actually injured in such a case. In my case, I work around my DC power supplies with pretty casual regard up to around 20 volts; my little "inner voice" starts talking in the 30-40 volt range, and I'm pretty cautious at the 40-50 volt level. Above that, I take the same precautions I use when working on line-powered stuff or higher voltages. The main technique is to connect test equipment with the power off, stand back, and turn the power on.
Another thought: even at "low" voltages (say, less than 40-50 volts), you can get a shock. A shock may not harm you directly, but your instinctive reflex action can cause you to e.g. jerk back. A friend once told me of how a friend of his got a pretty nasty cut requiring a bunch of stitches from a sharp piece of sheet metal in such a situation.
Sorry if that sounds like I'm a safety Nazi -- I'm not. But you also don't reach geezerhood like me if you don't pay at least some attention to the hazards...