Electrofuel makes the "PowerPad" line of supplemental (external) batteries
for many laptops. (Acer
It packs 160 watt hours of juice, which they claim is enough to run your average laptop 12 to 16 hours. All this, in a 1/2 inch high, 2.4 pound "pad" roughly the same dimensions as the base of a laptop. (8.75" * 11.75")
The "pad" is made of a "SuperPolymer" which seems simply to be a large Lithium Ion cell. The large capacity is further maximized by electrofuel's proprietary power management
They plan to offer souped up cell phone batteries in the near future. Applications in transportation, medical care and other fields are also planned.