Have you guys seen these? They are small portable hard drive sized units with enough power to jump start your truck a few times on a single charge.



heres the main seller:
http://antigravitybatteries.com/microstart/
I see there are quite a few others on the market as well and they all seem to contain the same flashlight optic and similar features. Its a Chinese product being re-sold by the American company so the "clones" may in fact be the same guts. Some apparently come with different cells though. I believe the micro-start has lipos and one of the clones has Lifepo4. Both of those have pro's and cons so its just a choice I'd say.

This video shows you a guy doing this a few years back with just a hobby lipo
http://www.youtube.com/watch?v=iflMpvM21Pk

They look pretty handy for the money ($80-140) and the things I like so far are:
-smaller and lighter then a good set of jumper cables
-using fairly robust/dependable technology (id think cell failure rate would be low)
-Priced similar or lower then most lead/acid jump packs
-requires charge only twice per year
-Charge your cell phone too in the event of dead car battery.

Cons I can see would be:
-low AH rating on them compared to true dual battery set up.
-at least one manufacture requires removal after vehicle start withing 10 second meaning its not shielded from the alternators charging current (lipos cant be charged incorrectly or they catch on fire)

Seems like a nice new middle ground between clunky large lead acid jump packs and the 100% redundancy of secondary battery set ups.