JJ's ESP8266 Ramblings
Originally aimed primarily at serial-over-wifi applications the ESP8266 is a SoC (System on a Chip) based on the Xtensa LX106 processor. It's overtaking the Arduino as a the hackers' choice for Internet of Toasters, sorry, Things applications.
Loosely related to the MIPS it's a 32-bit RISC processor running at 80MHz with 64KB instruction RAM, 96KB data RAM and up to 4MB of external flash. It has integrated serial, 16 GPIO lines with SPI, I2C, I2S support, a 10-bit ADC and 82.11b/g/n WiFi with a comprehensive IPv4 stack. And that's all the copying from Wikipedia I'm going to do - Google will help you find anything else you need.
For true IoT applications the current consumption is low, especially in sleep mode, so the simplest modules can run for over two years on a pair of AA batteries. Note that it's all 3.3V so it's not a drop-in alternative for mainstream Arduinos (Uno, Nano) and level-shifters may be required.