The cool thing about android is the number of interesting applications that are available from Market they may be simple but do interesting things. Here is one such application that I downloaded and installed recently.
Locale is an application that can customize your phone based on your location, time and many other parameters. For example you can automatically send out a tweet when you are at a certain location. You can change the screen brightness, ringer and other stuff.
From what I have seen till now, I like this one. Let me give it a try for couple of more weeks before I pass judgement on it.