Android (4.0+): Not to be confused with Tasker itself, Mini Tasker is another automation app for Android that can essentially "Do X whenever Y happens". The difference is that Mini Tasker comes stocked with recipes, it's really easy to use, and it's still remarkably powerful.
Mini Tasker (which, let's face it, will probably have to change its name soon) can do just about everything you'd expect an automation tool to do: it can start a specific media player app as soon as headphones are connected, fire an SMS message whenever you're within a specific GPS location (that you can pick from an overhead map), set an alarm or change the volume at a specific time of night so you can sleep and so on. You can create these actions from scratch, or you can head into Mini Tasker's "Task Market" and grab recipes other people have created already and customise them for your use.
The app's description at Google Play lists in detail all of your phone's actions that Mini Tasker can control, but they include most functions you're familiar with, including controlling your calendar, setting alarms, sending SMS, controlling Wi-Fi, NFC, GPS, and other data services, launching apps and silencing calls. They also list some of the conditions you can set as triggers for those actions.
The dev has gone out of his way to make the app as battery-friendly as possible, and he explains why the app is free (it was a university project).
Hit the link below to give it a whirl — the learning curve isn't as steep as some other apps we've seen, and so far it's been well received at Google Play.