One of the most useful functions is identify( ... ), and this is another hidden gem, as I continue to forget the name and I cannot ever find the name of the function in any Google search.
Given a floating-point number, identify(...) tries to determine if that floating-point number is an approximation of a number in closed form. The examples from the help page are more than sufficient to understand the applicability and usefulness of this function:
[> identify( 3.142857143 );
$\frac{22}{7}$
[> identify( 3.146264370 );
$\sqrt{2} + \sqrt{3}$
[> identify( 3.141701399 );
$\textrm{arcsinh}\left( \frac{231}{20} \right)$
[> identify( 3.141592654 );
$\pi$