wireless temperature sensor
Not wifi but a standalone XBee + temperature sensor will do this for around $30-$40 or so. 
The XBee has a uC + A/D converter and could read an analog sensor. At a 0.1% duty cycle (transmitting for 1 second every 1000 seconds) you can get around two years on two
AA's with a fairly simple design.

I have a short ZigBee writeup at
http://wiblocks.luciani.org/white-papers/intro-to-zigbee.html