SDK

pfalcon/esp-open-sdk: Free and open (as much as possible) integrated SDK for ESP8266 chips
espressif/esp_iot_rtos_sdk: ESP8266 new SDK based on FreeRTOS

Basic

ESP8266 Community Forum
mikrocontroller.net: ESP8266
Wlan2Serial Modul für 5 euro
New Chip Alert: The ESP8266 WiFi Module (It’s $5)
Tutorial for ESP8266 Serial WiFi Module
ESP8266 VirtualBox Setup (Ubuntu)
ESP8266 Test Board

Programming with Arduino IDE

github: Arduino IDE
WiFi Kochbuch mit ESP8266 (11.12.2014)
Arduino IDE mit dem ESP8266 (08.04.2015)
Adafruit HUZZAH ESP8266 breakout – Using Arduino IDE

Aliexpress

10 x ESP-12E
10 x ESP-12F (clone?)
1 x ESP-14

esp8266.com Community

Modules Family

AI Thinker

Espressif

ESP IoT SDK v1.3.0 08.08.2015
ESP IoT SDK v1.4.0 18.09.2015
ESP IoT RTOS SDK v1.1.0 21.08.2015

cadence IP: Xtensa

Wikipedia: Tensilica
Tensilica Xtensa Customizable Processors
Xtensa LX Overview Handbook (PDF)
Xtensa – Instruction Set Architecture (ISA) (PDF)

Wiki

github.com/esp8266/esp8266-wiki/wiki

Source Code Examples

github.com/esp8266/source-code-examples
WeatherStation Kit

NodeMCU

NodeMCU
github.com/nodemcu

2 x NodeMCU v3
1 x NodeMCU v3

WeMos

WeMos Electronics
D1
Get started in Arduino

https://github.com/wemos/WeMos_Boards/raw/master/package_wemos.cc_index.json

ESP8266 Node MCU Setup
NodeMCU Variants
nodeMCU with Lua and its native tools

LoLin / NodeMCU v3

Comparison of ESP8266 NodeMCU development boards

So, what’s with the V3? NodeMCU haven’t released a new specification so far. Hence, there’s no official 3rd generation board. Turns out that V3 is a “version” invented by producer LoLin to signify minor improvements to the V2 boards. Among others they claim their USB port to be more robust.
If you you compare the pin layout there’s only a tiny difference to the V2 layout. LoLin decided to use one of the two reserve pins for USB power out and the other for an additional GND.