Some "portable" headphone amplifiers aren't all that small, but the Firestone Audio Fireye Mini is downright tiny.
It's just 1.5 inches by 1 inch by 0.5 inch, and the soft-rubber-shelled amp weighs almost nothing, so you can hang it off your iPod, iPad, iPhone or any device with a 3.5mm headphone output. Plugging in a headphone turns the Mini on and lights a bright blue LED, unplugging turns the amp off. Charge the Mini via the USB connection and it'll play for up to 24 hours. It's available in green, gray, purple, red, and white.