If This Then That, or "ifttt" for short, is a site that allows you to set up various automated tasks online. For example, you could get a text message if the weather changes, schedule tweets and blog posts, or have anything you put into Dropbox also go to Google Drive. This is really convenient for making various online services work together. There are countless combinations to choose from, and it's easy to create your own "recipes" as well. All you need to do is create an account at ifttt.com to get started.
It's also very easy to set up some simple home automation with IFTTT using the Philips Hue lightbulbs or the WeMo from Belkin. These devices make it possible to blink a light when you get a text message or email, or even change the color of your lights to match weather conditions. More practically, you could have your lights turn on automatically at sunset, and back off again at sunrise every day. The WeMo makes it possible to turn other things in your home on and off from afar.
For more on setting up home automation, watch episode 36 of Know How, which demonstrates using IFTTT with the Belkin WeMo. Leo also talks about using IFTTT to automate Tumblr posts for a caller on episode 1016 of The Tech Guy.