Teensy-LC (Low Cost) by Paul Stoffregen is a powerful 32 bit microcontroller board, with a rich set of hardware peripherals, at a very affordable price:

Teensy LC – OSH Park Edition $11.29 USD

Teensy-LC delivers an impressive collection of capabilities to make modern electronic projects simpler.  It features an ARM Cortex-M0+ processor at 48 MHz, 62K Flash, 8K RAM, 12 bit analog input & output, hardware Serial, SPI & I2C, USB, and a total of 27 I/O pins:

Teensy-LC maintains the same form-factor as Teensy 3.2, with most pins offering similar peripheral features:

• Real Hardware Serial
• Efficient USB Communication
• Hardware Timers
• Analog Input & Output at 12 Bit Resolution
• 5 Volt Buffer For WS2812/NeoPixel LED Projects

Compared With Teensy 3.2:

Some features of Teensy 3.2 simply aren’t available at this lower price point.

• The I/O pins are not 5V tolerant.
• The maximum speed is only 48 MHz, and the Cortex-M0+ omits M4’s special math instructions.
• CAN bus is not available.
• Teensy-LC has only 4 lightweight DMA channels, rather than 16 fully featured channels.
• Serial1 & Serial2 have only standard buffers, not FIFOs.
• Fewer hardware timers are available.