Here are some useful articles on electronics and microcontrollers.
Digital I/O expansion methods.
How to use the TCS230 (/TCS3200) Color detector chip and easily add it to any of your projects.
With the ADXL345 acellerometer you can detect up to 16g! You can also find out how to use it for tap detection and more.
HMC5883L - How make a digital compass, Find out the differences between the HMC5883L and the QMC5883L and whether they are compatible.
Easily use an ESP8266 with the Arduino IDE and program your first sketch into the ESP8266
The MCP4725 chip is a 12 bit DAC with memory that outputs voltage that you can use for many dfferent purposes. Find out what they are in this page.
PCF8591: A four input ADC with single DAC. How good is this 8 bit ADC, and should you use it in your next project?