A clever former Fender amp designer merged a cigarette pack with a practice amp to form the Smokey Amp. Notable for its portability rather than its sound quality, the amp is powered by one battery that gives you ten hours of play-time. The amp is fairly loud for its size, so if you're in the habit of pulling out your axe to give impromptu serenades to people on the subway, £24.95 ($45) will get you what you need.
Smokey Amp [Firebox via Tech Digest]
DISCUSSION
I really like this project, though I don't smoke cigs.
I don't know where to find them for projects, but it strikes me that those new-generation microdriver type speakers might be good to excellent in a project like this.
(Sorry, I'm a modder.)