A guitar amp in your pocket
Vox is a guitar amp. Unlike most guitar amps, it's about the size of a deck of cards. Just plug the $40 box into your guitar and go. There's even an aux-in socket for mixing in another audio source.
Vendor ThinkGeek insists the results "perfectly reproduce the complex and warm distortion that is distinctive of a vacuum tube." There are three editions, one of which emulates the Vox AC30 amp, another that of classic British rock, and "metal," which "delivers the explosive roar of US high-gain metal sound."
Let's just wrap this up now, before I run out of scare quotes.
amPlug VOX Mini Guitar Amp [ThinkGeek]

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Scares my Firefox so bad it won't open the page due to some compression error or whatnot.
My co-worker brought one of these in to the office the other day. He said it sounds pretty good, and he's mixed/produced a few dozen bands so he has a pretty good ear. He also said when his Mesa blew a couple tubes he was able to keep practicing with the band by running it through the PA, though obviously that didn't sound as good. He has the metal version and gave it a solid review.
See also Smokey Amps, Altoids Amp.
"This Word Just In: Pete Townshend sterilized by Pocket Amp. Details and YouTube clips at 11..."
I've have a smokey 'amp' for about 10 years. The cool thing is that it's the size of a pack of cigarettes but you can also attach it to a full size cab and get decent sound....
These things have been around for ages. The ones posted here aren't particularly anything special, though the Vox one gets a decent sound. Smokey Amps and Rockmans are where it's at with pocket amps.
I have the metal one and it's great. It plugs right into the guitar, unlike the mini Marshall amp that has a cord.
is it Snake Oil week and I missed the memo?
these things are pretty sweet. they sound great, i tried out the brit tube one and the metal one.
it's too bad they don't make as much awesome stuff for bass as they do for guitar.