The ESP8266 is similar to Arduino, but smaller with built-in WiFi, and can be had for under $10

Find Your ESP8266 On eBay!

Simple Auto Shut-off Circuit For Any 12vdc Dumb Charger

Learn How To Setup A Raspberry Pi RFID Reader

Make Your Own Wallet Size LED Resistance Calculator

5 Raspberry Pi Operating Systems That Aren’t Linux

Build A Wireless Electricity Transmission Circuit

Make Your Own 9V Battery-Powered USB Charger

Use the Arduino IDE to program the ESP8266.

Arduino compatible dev board with built-in Wi-Fi for less than $10?

Battery Desulfator: A very simple capacitive battery charger made from scrap parts

30 LED Projects

Use a $20 DVB-T TV tuner dongle based on the RTL2832U chipset as a Software Defined Radio (SDR)

Make Your Own Solar Powered Outdoor Night Light

DIY Battery Charger. This one is a take-off of the $3 Charger posted below but used for smaller AA, AAA, C, or D cell batteries.

Make a Joule Thief

200 Transistor Circuits Book One (1 to 100)

200 Transistor Circuits Book Two (101 to 200)

The $3 Battery Charger