The Arduino family of products has become very popular; by combining the open source nature of the hardware and software, the Arduino foundation hit a homerun. With millions of Arduino-based devices, there are also tons of projects, products, and resources available that expand the dev board's potential. But with all this hype around the Arduino-based boards, let's not forget about the other great Arduino alternatives. There are numerous boards that use AVR chips similar to the Arduino, but this list is for boards without AVR cores. Check them out!
For around $10, the Ti MSP430 LaunchPad is a great low-budget alternative to an Arduino. The MSP430 LaunchPad can be used with 3 Ti supported IDEs: Energia IDE, CCS Cloud, and Code Composer Studio. Energia is visually extremely similar to the Arduino IDE and comes with quite a few example programs including "Getting Started with Energia" and "The MSP430 is Very Easy." Like the Arduino platform’s shields, the Launchpad platform has various "booster packs" to add functionality to the board.
The Netduino is an ARM-based development board programmed with the .NET framework. With up to a 168Mhz processor, these boards are pretty powerful when compared to an Arduino UNO. One of the biggest benefits of using a Netduino coming from an Arduino is a similar pin out on many of the Netduino boards. There are several different variants of Netduino boards-- take a look at them and see if any fit your needs.
The Teensy line of development boards are small boards based on a Freescale ARM Cortex-M4 microprocessor. With a clock speed of up to 75Mhz, multiple data busses, 5v tolerant pins, and a reasonable price, the Teensy offers a lot in a small package. The teensy uses the Arduino IDE, so a lot of your code will work without much changing. Additionally, there are some add-on boards for the Teensy to increase its functionality.
Originally started as a Kickstarter under a different name, the Particle with WiFi capability is starting to gain popularity. The Particle’s 120Mhz ARM Cortex M3 is programmed using the Photon’s cloud-based IDE, which offers some extra functionality for your next IOT project. Photon is expanding their product line with several shields and also has a 3G variant of the board, the Electron, due for release at the end of January, 2016.
The ESP8266 isn’t a dev board, but rather a microprocessor with WiFi capability that is becoming very popular. ESP8266 development boards start at $2 and numerous different types are available. The ESP8266 can be programmed with the Arduino IDE and has a lot of documentation available. For the price, this development board is tough to beat--just be careful though; these boards typically take 3.3v, not 5v!
While this is not a complete list, there are dozens of developments boards that can definitely be useful. If you think I left an important development board off this list, let me know in the comments below!