Odseven ESP-01 ESP8266 Serial Wireless WIFI Module Transceiver Receiver for Raspberry Pi
This interesting module is a lot of fun for hobbyists and students who are interested in experimenting with the ESP8266 WiFi chipset. We bought a bunch of these modules, updated the firmware to the much-easier-to-use v0.924 and wrote some Arduino code to grab a webpage.
It doesn't have 5V to 3V logic level shifting so you'll probably want to pick up a logic level shifter, it also doesn't have a regulator on board and it can use big spikes of 300mA or more current at 3.3V so if using with an Arduino, an external 3V regulator is essential! (Seriously don't try to power it from the 3.3V power pin the Arduino!) Also, the connector is kind of annoying and not breadboard-friendly, some female-to-male wires can help out in that case to make it easy to plug into a breadboard.