This evening, I saw a reference to something called an ESP8266. A quick web search revealed it was a $5 WiFi chip that was mentioned last August 2014 on Hackaday:
As of this writing it has only been about six months since the discovery. At the time, little was known about it beyond some documents in Chinese and some early attempts to write code to use it. Today, however, you can find this part on Amazon for $7.99 with Prime shipping, or much cheaper on e-Bay with slow shipping from China:
The chip can been hooked up to a UART (transmit and receive), or via SPI. This means, with a cheap TTL-to-RS232 adapter and a power supply, you could WiFi enable anything with an RS232 port with a bit of communications software.
The next time I have a few spare bucks, I plan to order one and see how easy it is to work with. It could be a fun way to WiFi-enable a CoCo or Arduino :)
And for those curious, here is a tutorial on using it I found:
Please leave a comment if you have worked with this chipset. It seems it might not take much to make a C library for Arduino that mirrored the official Arduino WiFi shield API, but used this inexpensive part.