A couple of guys from MIT have started Supermechanical and developed a WiFi module that can connect sensors to the Internet. According to their project listing on the KickStarter website, "Twine is the simplest possible way to get the objects in your life texting, tweeting or emailing. A durable 2.5" square provides WiFi connectivity, internal and external sensors, and two AAA batteries that keep it running for months. A simple web app allows to you quickly set up your Twine with human-friendly rules — no programming needed. And if you're more adventurous, you can connect your own sensors and use HTTP to have Twine send data to your own app."
Twine is a wireless module tightly integrated with a cloud-based service. The module has WiFi, on-board temperature and vibration sensors, and an expansion connector for other sensors. Power is supplied by the on-board mini USB or two AAA batteries (and Twine will email you when you need to change the batteries).
For $99 to support the project development, you can monitor temperature and motion with one module and receive a text message, Twitter or e-mail if the conditions you set are exceeded. There is a simple web interface to setup the module and conditions of your choice with each sensor. The WiFi module comes with a built-in temperature and accelerometer and you can add other external sensors for things such as moisture, magnetic switch, etc. which you can add via an external connector board.
Applications include sensing water in your basement, opening of doors, washer/dryer is finished, etc. They have already exceeded their funding goal by a huge margin but you can order the various configurations on the KickStarter website for delivery in March of 2012.