Portable 5V power supply
What you need is an efficient buck-boost converter that will reduce the voltage when the cells are above 5V, and increase the voltage when the cells are below 5V.

This is important to maximize the useful energy that the cells can provide. A fixed linear regulator can never fully utilize the stored charge in the cells since the input to the regulator must always be higher than the output.

As an example, Texas Instruments has a series of high-efficiency buck-boost converters that are intended for use with rechargeable cells.