Nano Power Timer is just under $6 and makes it easy to take just about any microcontroller project (Arduino or anything else) and effortlessly give it a hardware low power mode, all without needing to mess with any low-level microcontroller programming.
Here is how it works: the switches on the board set a time delay. During this time, the Nano Power Timer sips barely 35 nanoamps but the catch is that power to your circuit is completely cut off.
Once the timer is up, power is applied to your circuit and it will turn on to do its job (read a sensor to decide whether or not to do something more, for example.) Once it is finished doing whatever it needs to do, a GPIO pin signal to the Nano Power Timer's DONE pin is all it takes to go back to sleep and reset the timer to do it all over again. It works on any voltage between 1.8 and 5 volts.
Low power operation is the difference between viable and non-viable for some projects. Not everything can be plugged into the wall all the time and not everyone can easily make low-power modes work. A board like this is a great companion to a solar charger solution, as well.