The Ibanez IBZ15GR is a small rehearsal/practice guitar amp with 15 watts of power and an 8" speaker. It's light, affordable, and almost everything you need in a portable amp, including tone.
The IBZ15GR is designed for electric guitar, but you could use an acoustic guitar also. Other features include a closed-back cabinet for increased bass response, a separate Distortion channel with Gain and Volume controls, and reverb. There's also a headphone jack for private practice (and to keep others off your case), and an aux input for your iPod, CD player, etc.
Turn on, plug in, get happy!