Took part in the Particle (formerly Spark) Kickstarter some months back and picked up a neat wifi, cloud-enabled, Arduino-like micro-controller (phew).
Hooked it up to a DHT-22 temperature and humidity sensor, and placed it in my living room. A node.js app polls the Particle core for data every minute, and stores that data in a PostgreSQL database.
Hooked up C3.js to visualize the last 8 hours of data. You can see spikes when I open the sliding door to the outside and the indoor humidity and temperature change. You can also see the cycle of the air conditioning, turning on when the indoor temp gets too high. We leave it on right now to keep our dog Gumby cool in the recent 30 degree heat.