I was looking for a headphone guitar amp with the following features:

• Integrated guitar plug: Doesn't require an extra cable or have a box flopping around.
• Rechargeable battery
• Flexible set of basic effects with decent quality: distortion, reverb, amp simulation, etc.
• Built-in metronome

Although there are commercially available headphone amps that support a few of these, surprisingly, none supported all of them, so I decided to build my own. The heart of this is an ARM microcontroller, which processes the audio signal and applies digital effects. The hardware consists of a custom printed circuit board in a 3D printed enclosure.

TBH, I kind of ran out of steam on this project. Pictures of the prototype I built are in the Wiki: https://github.com/jbush001/HeadphoneGuitarPracticeAmp/wiki/Prototype-Pictures, but I didn't end up bringing this up.