A guitar headphone amplifier is a great little mini amp, especially designed for practising your guitar. Plug in your guitar, and your headphones, switch on and you're away. Fantastic if you're stuck in the hotel room, or travelling light. Not to mention, if you can't afford a full size amp, then this would make a great way to get started.
Different makes and models
These little mini amps come in all different flavours, and by several manufacturers. Namely Vox, Marshall, Fender, Behringer, C=Tec, Line 6, and Dean to name a few. There;s a whole list of them out there if you search. The great thing about some of these Headphone amps is the fact they have emulated the sounds of their full size amps.
The Vox AC30 for example is available in three styles: Classic Rock, and Metal. With a added AUX in jack lets you jam along with your CD/MP3 player. Neat.
There's also some bass versions, so you bass players won't feel left out. The Vox AmPlug Bass is modeled after VOX's classic AC100 bass amplifier, so you'll get some great tones out of this little amp.
If you want to take things up a gear, then you could also consider the Line 6 Pocket POD, This little guy really packs a punch. Portable Guitar Effects Processor with 300 Presets, 32 Amp Models, 16 Cab Models, 16 Effects, Audio Player In Jack, USB Connection. You can use the Pocket POD as an effective practice tool with headphones, and audio player, or as a full-blown multi-FX processor with your amp. A USB connection is onboard, and you can access free editing and tone library software online.
Check out the youtube video's below, and get a idea of what these little headphone amps can do.