Planned features

ESP8266 ESP-07 module as core (802.11 b/g/n WiFi, 32-bit low power CPU, Arduino IDE programmable)
LiPo-battery powered using LTC4080 as charger and power management
Small enclosure (1551H by Hammond)
50x29mm PCB (to be <50x50mm for cheaper manufacturing)
DS18B20 and other optional sensors (i²c, OneWire, ...) e.g. TSL2561 or DHT22 and many more...
optional USB serial using the cheap CH340G
optional SD-Card in SPI mode
Arduino "standard" programming header as backup/extension
...
...

History

Plain prototype for software testing (just the ESP8266 ESP-07, a DS18B20 temperature sensor and a cheap AMS1117 voltage regulator):

Deep sleep current consumption (power LED from the ESP-07 has been removed to save energy) only 24µA (after the AMS1117 voltage regulator):

Got U. Rüegg to work with me on this project because developing together is more fun and having some review is always nice :-)

The gerber files sent to the manufacturer: 20150720-ESP-WiFiTemp-gerber-as-ordered.zip
Some rendered images:

(rendered using gerber-viewer.com)