Here's my second headphone amp built about two years ago. I used a Radio Shack 4AAA battery case. It uses two lm386 amps (I know they are jellybean chips but at low output currents they still sound pretty good). I added a low pass filter on the input for added bass (I was going through a period where I really liked powerful bass). I've added a max756 to step up the 1.5V to 5V (battery life is pretty good at around 15+ hours). I was planning on adding a digital volume control (ds1802, and hence the two buttons on the side) but ran out of room. My next revision for a pocket amp will be in the same case, but will feature a custom etched smt pcb and digital volume control.
This one was my first headphone amp I ever built (made it freshman year). The case is a recycled plastic hair pin container that I "borrowed" from my mom. It uses the same chip as my desktop amp above, runs off of a nine volt, and has an analog volume control. The power supply splits the single source using a resistive divider and buffers and stabilizes the output with the help of a opamp. Not my best work, but it sounds pretty good and I learned a lot making this.