The ThinAmp from AXL,
introduced in January at NAMM, is a portable amp packed into the size
of a laptop: only 2" thick and just 5 lbs. (!) You can literally fit it
into a laptop bag — in fact, you can probably fit it and your laptop
into a laptop bag. The amp is geared at guitarists, but Synthtopia reviews it and finds it suitable for laptop, too.

Of course, you can't expect a whole lot of sound out of a 2" amp; while
it has EQ and DSP presets for reverb, vibrato, flanger, chorus, and the
like, the ThinAmp's sound will be a bit thin with just 10W of power. On
the other hand, for US$140 street and at this size, this is pretty
tempting.