The TrekLight PackBack weighs just 85 grams and can fold into a package the size of a toddler's fist, but its uber-strong parachute nylon construction means it can hold over 13 kilos of stuff. Forget a fist, you could fit a whole child in there!
Its feathery weight and packability do come with some drawbacks, the main one being ergonomics. Don't expect any sort of padding and god help you should you need a hipbelt. That being said, it does have a secondary pocket on the outside. Also when you expand it out, the stuffsack turns into a secret little inner pocket that dangles in the main pocket. It's the perfect size for certain unmentionables.
It retails for US$27 but unfortunately, TrekLight sold out of these as soon as they announced them, but another production run is in the works (expected in a few months). Until then, continue assuring your current backpack that the keychain you got it totally has a nice slimming effect on its figure. [TrekLight via GearJunkie]
Images courtesy of GearJunkie