Turn Laptop Battery to a Power Bank
It's not impossible, but there are some traps:


first you have to decide whether you're going to open it up (voiding the warranty, slightly hazardous) or use it unmodified. If unmodified you have to find a matching connector which may be proprietary.
Some batteries are "smart" and rely on a digital negotiation with the host laptop before turning on the power. See http://batteryuniversity.com/learn/article/how_to_repair_a_laptop_battery
Lithium batteries are only good for a limited number of cycles. More use will wear them out more quickly.


Once you've addressed those the battery will put out usually 10-15V. A buck converter down to 5V will let you charge USB devices from it. However, realistically it's cheaper to buy a Chinese power pack designed for the purpose.