Sometimes, important calls come at inopportune times, like when you're in a meeting or deep in face-to-face conversation. What do you do when you can't pick up, but don't want to blow the caller off? Android has you covered.
If you have a device running Gingerbread, Ice Cream Sandwich, or Jelly Bean, the “incoming call” screen will give you three response options: pick up, ignore, or "ignore with text." The way to select that option varies from phone to phone (on most Samsung phones, you slide the call icon up rather than left or right), but the result is a list of canned text messages like "Can't talk now. Call me back in five minutes?" You can even include custom messages on this list to send to your caller, presumably averting uncomfortable “Why didn't you pick up your phone?” discussions later on.
If you're running Froyo, check out the free “Incoming Call Plus (beta)” app. It'll give you the same feature, although you'll have to put up with a different-looking “Incoming Call” screen.