From Stereo Times comes a review of a great little headphone amp. As you can see, it's battery powered and small enough to tote with your iPod. Ray Samuels of Emmeline also makes a series of great sounding headphone tube amps for home use as well as this little gem that runs $395. Eujin Hong pronounces the SR-71 a winner.

Also of interest, Ray Samuels recommends the Shure E5c headphones for his amp. When the manufacture recommends something, you know it's got to be good.

Emmeline SR-71 Headphone Amp [Stereo Times]
Emmeline Audio Home [Emmeline]
Shure Headphones [Shure]