Countries listed alphabetically by region.
Austria: Physical Computing
Serbia: Saperel d.o.o.
Estonia: ITT Group
Germany: Segor-electronics (search for "Arduino"), ElectroDrome, Watterott Electronic, Elektronikladen | ELMICRO, TinkerSoup, reprap source, Bausteln, Antrax, EBG Darmstadt (search for "Arduino"),Fritzing Shop, Komputer.de
Lithuania: UAB "IDOIN"
Spain: Libelium Cooking-Hacks, BricoGeek, e-lioness, TuxBrain, Cana Kit, Electan, Componentes Escobedos, OpenDomo, Voipminic, ardutienda, Microlog, Albacete Digital, Innova Didactic SL, Cetronic, ERA ROBÓTICA S.XXI S.L.U
United Kingdom: Cool Components, SK Pang electronics, oomlout, Robotiq, RobotBits, Proto-PIC, Earthshine Electronics, Hot Solder, Technobots, DTECH, Love Electronics, Domit, Active Robots Limited, Phenoptix, http://www.hobbytronics.co.uk/!HobbyTronics
Chile: Ingeniería MCI
USA: SparkFun, Maker Store, Adafruit Industries, Little Bird Electronics, Modern Device, FunGizmos, NKC Electronics, Gravitech, RobotShop, Liquidware, Hacktronics, MakerBot Industries, Microcontroller Pros, Curious Inventor, AeroQuad, CuteDigi, EIO, Teach me to make, UltiMachine, Electrojoystick.com, Electronics is Fun, AME - After Midnight Engineering, Trossen Robotics, Jameco, Zagros Robotics, Advanced Micro Circuits Corp, iHeartEngineering, 3D Robotics, Jaycon Systems, Elexp, Abra-electronics, Reuseum,
Asia and Pacific
Nigeria: Satajanus Bots
SmartProjects handles the manufacturing of all the Arduino products, with the exception of the Arduino LilyPad, Pro, and Mini Pro from SparkFun and the Arduino Nano from Gravitech.
Note [from massimo]:We stress the fact that the Smart Projects boards are made in italy because in this globalised world, were getting the lowest possible price for products sometimes translates into poor pay and working conditions for the people who make them, at least you know that who made your board was reasonably paid and worked in a safe environment. (this obviously applies only to the boards marked "made in italy", we can not attest to the manufacturing process of "third party" boards).