DBPower battery booster
I'm very impressed with this thing. It was recommended to me by a colleague, I found a nearly-new one on Ebay and it's been under my passenger seat for six months. This morning, my car battery (less than a year old!) was mysteriously too flat to start it, but the booster had it going within a minute and spared us a really annoying Sunday morning delay which would have disrupted ten people. Really simple to use, and it has USB for charging phones too. That said, it did drop 60 percentage points of power after starting my Golf GTi, so I wouldn't over-use it for other purposes.