I just bought one. I'm planning a fairly long trip and thought it might come in handy.
The jump starter is a lithium battery
with short jumper cables. If your battery
is dead, you just hook this up and jump your car without needing a 2nd vehicle. The one I bought also has a built-in flashlight and power ports for recharging cell phones, etc. Should be some great deals between now and New Year's.
They come in different amps, so you need to get one rated for your vehicle. Mine is rated at 800A, Peak 18000mAh (Up to 7.0L Gas or 5.5L Diesel Engine). It's rechargeable from 110 vac or 12 vdc and is small enough to fit in glove box. This is a picture of the one I bought, but there are many companies making them.