Over at the Magic Cafe, there's a great thread about divining phone numbers.

The second method described in this post uses simple algebra to let you find out someone's phone number. An explanation of the underlying algebra is also included.

While I'd seen the algebraic method before, the first version described in that post, which is described more clearly in this post, is one that I'd never run across before. The basic idea is that of the classic number cards trick (Java required), but applied to larger numbers in an ingenious way. One of the benefits of this method that it doesn't matter how many numbers are in your country's phone number system, it will still work.

Of course, presentation is important, so McCritical provides a great presentation, which helps hide the mathematical nature of the effect.

As a quick shameless plug, those who enjoy the above routines may also be interested in Mental Shopper and the Serial Number Feat, too.