In principle, any chemical reaction that is spontaneous can be carried out in such a way that it makes a voltaic cell. Even a process such as the dilution of a concentrated acid can be harnessed in a voltaic cell.

Fruits (lemons, oranges, bananas) make excellent sources for the electrolytic conductor in an electrochemical cell constructed from two different metals. This makes for a nice demonstration; it won't produce much power, so you can't do too much more than drive a digital meter, calculator, or other high impedance device with it.

Zinc-Copper Battery (with Salt Bridge or U-tube). Nernst Equation: The Dependence of emf on Concentration.