The Digispark Pro Ecosystem is the cheapest, Arduino compatible development platform for Mobile and Wireless hardware development.

The Digispark Pro is designed to compliment the Digispark - it's the middle child between the Digispark and the full blown Arduino. It is even easier to use than the Digispark, even more compatible with standard Arduino examples, and packed full of i/o, more program space, and more features! With new shields and libraries the Digispark Pro is also the most affordable mobile hardware development platform! And it is still just as small as a Digispark, and just as affordable - because electronics should be accessible to all!

Specifictions:

Compatible with Arduino IDE 1.5 (OSX/Win/Linux)
Fully signed drivers and executable for easy installation
USB programming, USB device emulation, USB-CDC virtual serial port emulation
16 Mhz AVR MCU - using a true 16Mhz precision crystal
16KB Flash Memory (14.5K+ after bootloader)
Serial over USB debugging and communication!
14 i/o Pins (2 shared with USB)
I2C, true SPI, UART, LIN, and USI
ADC on 10 pins
3 PWM Channels (which can be assigned to a selection of pins)
Power via USB, or External Source - 5v or 6-16v (automatic selection)
On-board button that can be used as a reset, program, or user button - or can be disabled to use that pin as general i/o - without changing the bootloader
On-board 500ma 5V regulator
Power LED and Test/Status LED (on Pin 1)
User accessible solder jumpers to disable LEDs, and other features for lower power consumption
Two mounting holes.
Breadboard compatible pin out/spacing (the three side header pins are only for legacy shield support).

The Digispark Pro is the cheapest, ready to go, easy to use, mobile hardware development platform!

With Bluetooth Classic, BLE, and Wi-Fi shields and examples for Android and iPhone - and USB Serial (built-in) connectivity for Android (devices with USB OTG supporting Serial devices) .

USB Programming:

Programming the Pro is as easy as hitting the program button when prompted to
The Digispark Pro uses the newest Micronucleus USB bootloader for programming which allows easy programming over USB direct from the Arduino IDE (or command line).
Micronucleus is the official bootloader of the original Digispark and it has been installed on over 40,000 devices. It is a completely open source, community driven project, and has proven to be the smallest, most reliable, USB bootloader for the Attiny series of MCUs.

USB Debugging:

The Digispark Pro can emulate a USB Serial Port! This makes it as easy to use as an Arduino Uno for beginners/debugging!
The Digispark Pro supports emulating a USB CDC/Serial Device - when enabled it shows up as a serial port on all major platforms (OS/Win/Linux/Raspi/Android) - which means it will work with the Arduino Serial Monitor, other programs designed for Arduinos that appear as a serial port, and be much easier to integrate into custom programs!

USB Device Emulation:

Libraries available for the Pro to emulate a USB Keyboard, Mouse, Joystick, or Generic HID Device, This means it can appear to your computer as if it were that type of device - allowing you to easily have your device type, move the mouse, act as a joystick, and more!

Wi-Fi, Bluetooth, BLE, Mesh shields:

The Digispark Pro is backwards compatible with all existing Digispark shields - and it has some shields exclusive for the Pro including: a Wi-Fi shield, a Bluetooth Classic shield, a Bluetooth 4.0/Low Energy Shield, and a nRF24L01+ low cost mesh networking shield. These low cost shields make the Digispark Pro the cheapest way to jump into Internet of Things, Mobile, and Wireless development!
I2C, SPI, UART, LIN, USI: The Digispark Pro is ready to connect to just about any peripheral - it has stand alone SPI, a UART (with LIN capabilties), and I2C that is shared with a USI bus - which could act as a second SPI or UART for advanced users. This gives it all the bus options of an Arduino Uno and more!

Package Includes: