Android: You probably have a handful of contacts that you call more often than the others, and usually at predictable times. CallWho learns who you call and when, then adds those contacts to a handy widget.
The widget isn't just a list of the people you talk to most often. It tries to guess who you want to talk to right now. So, if you call your spouse every day at 6pm, it will place their face on your home screen around that time. It also weighs unanswered calls into the equation to let you know when you might need to get back to someone. The widget learns over time, so the longer you use it, the more accurate the suggestions should be.