This response may be too late, as it's the day after the GC sale, but I'm surprised no one has mentioned the Roland Micro-Cube. Runs on AC or 6 AA batteries, and weighs about 7.5 lbs. Has 7 effects models. I've seen street bands use this amp in subways, and having one myself, it really packs a punch sound-wise. Unless I've read the specs wrong, it's only 2 watts! Has headphone jack, audio out, built in tuner.

I liked the description of an ad I saw: "small enough to run out of your girlfriend's house with when necessary"- something like that.

Bought mine on sale at GC for $99.