Manufacturers
Arduino, Beagleboard, Espressif, Fluke, GW Instek, Raspberry Pi, Rigol, Siglent,

Retailers
Adafruit, Amazon, DigiKey, Maker Shed, Newark, PiBorg, RadioShack, Saelig, Sparkfun, Tequipment,

Information / Forums
EEV Blog Forums, Element14, Make,