DIY Linux portables - hardware (and some software)

I prototype and make portable Linux-based devices for fun and profit - from zero to a complete product. If you're interested in building your own Linux-powered portable helper, for whatever purpose, I have my own set of guidelines and tips that I'd like to share. What have I personally built so far?

• A portable 3G/WiFi/Ethernet router
• A password manager
• A cellphone
• A hardware hacking workbench
• A wearable music player

Plus, there are much more other devices that I haven't personally built, but I know and can tell about.