I thought this would be fitting to what we are doing in class right now. This link shows how to make a simple 5V DC power supply. The power supply is rechargeable via USB in your computer. Another bonus is you can install a On/Off switch. This project involves quite a bit of modding simple electronics but can be made pretty cheap.
I think this power supply could be use full, and is definitely convenient. One down side though is that it is only a 5V power, therefor you can’t use it to power heavy voltage required electronics.