Seeed Studios is currently advertising the Air602 WiFi module at a price of $1.90 for single unit quantities. The Air602 is tiny (smaller than a postage stamp and smaller than the ESP8266), but before you whip out your credit card to order half a dozen, you need to realize that one reason it’s so small is that there is no on-board antenna and no antenna socket, just a normal, edge connect pin on the board.
This thing in not an ESP of any variety. It is based on the WinnerMicro W600 SOC, which has a Cortex M3 core. The block diagram looks very impressive (although at the moment, the WinnerMicro web site is much less impressive and is doing an excellent impression of a honey-pot/tar-pit trap for unwary browsers), with dual UARTs, I2C, SPI and I2S interfaces, an RTC and hardware crypto all baked in.
The Air602 module has a limited number of pinouts (more than the ESP-01, though), so the available interfaces are fairly limited. There are 12 pins available, but with two ground pins, one 3v3, one antenna and one (input only) reset, that leaves only 7 pins available for I/O and those seem to be split between the two UARTs as primary (with the SPI bus as alternate use) and the single GPIO-8 available as unassigned. I’m guessing that the reason for the two pins being assigned to GND is an attempt to make it easier to add a micro antenna socket (the grounds are either side of the antenna pin).
The SDK zip is available from Seeed’s site and, given the current, sloooo-o-o-w state of the WinnerMicro web site, that’s where I’d advise you to go for more info. The module itself seems to ship with LUA and an AT command set implementation as standard (again, shades of the original ESP-01). Seeed also have a WiFi development board, based on the Air602 module, for only $2.90, which is a give and take proposition. It has an antenna (big plus) and the PCB is made to plug directly into a USB socket, but only 5 of the W600’s IO pins are available as through-hole connections.