The Devantech USB-I2C interface module makes it absurdly easy to access an I2C device through your USB interface
Handy device to run your USB equipment electrically separated from your PC. It removes a real potential problem when using USB-based test equipment!
Need to add a virtual COM port to your PC? The TTLyFTDI adapter plugs onto your USB mini-B cable, and breaks out all the standard lines for Arduino-type TTL communication!
Netduino Plus 2 expands on the impressive 32-bit open source Netduino by adding ethernet connectivity and an SD card socket.
Have all of the digital logic and analog circuitry necessary to implement a full-speed USB peripheral/host controller with your Arduino. Use it to interface and control any USB slave device - thumbdrives, digital cameras, bluetooth dongles, game controllers, Android phones and more!
The Arduino Ethernet is a microcontroller board based on the Arduino Uno, and incorporating a WizNet W5100 TCP/IP Embedded Ethernet Controller.
Here's the official Arduino Ethernet Shield With PoE Module
The Adafruit LCD backpacks reduce the number of pins needed to connect to an LCD. LCDs are a fun and easy way to have your microcontroller project talk back to you.
This board can do it all - breakout the Synapse RF module pins, give you a USB port/FTDI chip to program, supply it with a regulated 3.3V, integrate a reset switch (for updating & erasing), and also give you mounting options.