Tasker for Android lets your phone react to its current state or environment
It works by creating rules that question your phone's current "context" (or state): "Is the phone upside down?"; "Have you just missed a call?"; "Is your battery almost empty?" -- if any of these questions returns a "yes," then an action is carried out. That's where this app becomes magical: an action can be almost anything. Your phone can make a call, or send an SMS; it can alter the ringing volume during work hours, or silence it during the night; it can run apps, or react to when you plug in the USB cable or headphones. This is remarkably similar to context-aware patents currently dueling through the US patent system filed by Google and Apple.
The excellent FastCompany review walks you through a nice rule: if you place your phone face-down (say, in a meeting), it automatically switches to silent mode. They also suggest another awesome rule: tell Tasker to SMS "on the way home" to a loved one when your GPS coordinates reach the train station.
My nerdy brain is leaping to try some really crazy ideas, though. How about using GPS so that your laptop only unlocks if your phone is nearby? Or a dead man's switch that formats your phone if it gets too far away from your laptop -- or if you don't open a certain app by 5pm each day? Of course you could tell the phone to take a photo every 10 minutes so that any potential thieves are caught in the act! The possibilities are almost endless.
The app itself costs about $5 from the Android Market, but you can download a 7-day trial from the Tasker site (which you can later upgrade). The Tasker Wiki has lots of excellent guides and walkthroughs to get you started, and even some ready-made profiles for every-day tasks.