<p>Is there a specfic <a href="http://en.wikipedia.org/wiki/Design_Patterns" rel="nofollow noreferrer">Gang Of Four Design Pattern</a> that you frequently use, yet hardly see used in other peoples designs?  If possible, please describe a simple example where this pattern can be useful.  It doesn't have to necessarily be a Gang Of Four pattern, but please include a hyperlink to the pattern's description if you choose a non-GoF pattern.</p>

<p>Put another way:<br>
<strong>What are some good/useful design patterns that I, or someone else who does have a passing knowledge of the main patterns, may not already know?</strong></p>

Strategy pattern maybe? I see not a lot of person using it and it's quite useful when calculations change or can be accumulated together. I use it when a part of the calculation can be replaced by another calculation. Often in program that use for enterprise rate for product.
Here is some documentation :
Wikipedia
DoFactory