Tasker (Android app)

Lets you automate many activities on an Android phone. You define a context based on various conditions (e.g. connected to a Wifi network, using certain cell towers, phone spatially oriented a certain way) and various actions to perform upon entering and exiting that context. You can set variables and condition upon them, there is flow control for actions, customisable home screen widgets and shortcuts, and many other neat functions.

Some examples of tasks I use / am pondering:

• When my phone is on any of the cell towers around my house, it switches the wifi on (to connect to my home network)
• When the phone connects to my home network, it sends a magic packet to my PC and turns it on - but only if I am getting home at a time when I'm likely to want to use the PC (varies depending on day). But it only does this if it has been disconnected for at least half an hour (in case the connection drops out momentarily overnight)
• When my phone disconnects from my car's Bluetooth hands free kit, it waits two minutes and then switches the Bluetooth antenna off.
• When my phone is connected to its charger and turned face-down overnight (how I leave it next to my bed), it engages silent mode until 08:30am
• A toggle button on my home screens, that switches my phone to silent and turns network-level call forwarding on (for when I am at work/movies/etc)

There's a wiki with lots of downloadable setups you can experiment with for neat results.