While Arduino is an open source system, "Arduino" is a trademarked name and may only legally be used for products from the Arduino's originators or licensed suppliers. Arduino are quite clear about this requirement - [**see here**](http://www.arduino.cc/en/Trademark/CommunityLogo). 

As a result, compatible microcontroller boards are legally required to NOT identify themselves as being "Arduinos", so the term "Arduino compatible" is about the most that even an Arduino-identical clone can claim. 

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As karan notes, some Chinese sourced Arduino compatibles "have issues". 
however, I have had no problems at all with Arduino compatibles manufactured and supplied by [**this Chinese supplier**](http://www.aliexpress.com/store/812021/search?SearchText=nano). As well as the basic devices they sell various interface boards (shields) and related equipment. Prices are astoundingly low, free postage is included in many cases (about 2 to 3 weeks delivery time to New Zealand) or you can pay for courier delivery. I buy Nano or Pro-Mini compatibles devices - usually in batches of 10. As you will see - Nano compatible costs $US3 each in 10's. Pro Mini compatible is currently $US1.68 each in 10's. !!! 
Uno compatible [**here**](http://www.aliexpress.com/store/812021/search?SearchText=arduino) is $3.80 in 10s. The voltage requlator may be a different type than than on 'real' Arduinos (but work OK) and versions with a USB-serial bridge IC use a non FTDI IC and you need to source and load the correct (freely available) drivers. 

NB: (1) I have NO commercial connection with this company except as a very happy customer. 
(2) I'd expect you to have no problems with these or other products from them but I of course have no responsibility for what they sell.