Android: If you receive many text messages or calls at inopportune times, like when you're driving, Auto SMS might be very useful for you. Not only does it send auto replies, the free app also schedules and reads messages to you.
You can set a number of profiles for this Android "out of office reply" app to set the message and duration of the profile. For example, the profile for when you're driving could send a text with "Sorry, I'm in traffic" to all your callers or texters. There's also a handy auto reader feature that reads your text messages to you when you're driving.
In addition to the auto responses, Auto SMS also automates other aspects of text messaging. If you want to make sure you don't forget to text your friend "happy birthday" some day next week, you can schedule that. Or if you just want to create text message templates or individual texts for future delivery (even recurring ones), you can do that too.
Although Auto SMS lacks GPS or Bluetooth or other contextual switching, it's just one tap to from within the app to turn an autoresponse profile on or off, and for even quicker access you can set up widgets on your home screen for different profiles or the auto reader. Auto SMS has a lot of features for dealing with text messages and calls when you should be focused on driving safely or just don't want to be disturbed.