The function uisetcolor is very useful, but not easy to find. docsearch color found it at least under over 700 of other matches in R2009a and in 2011b it is found as 22th result when sorted for the relevance. But tricks like docsearch set ui color do not work and if a user has a lot of toolboxes, it is unlikely that he reads the complete documentation to find it by accident.
Do you know more such documented gems, which are hard to find, e.g. due to missing cross links in the "see also" lines? How did you find them?
Undocumented gems, which I did not mean here, are investigated and discussed at http://www.undocumentedmatlab.com/.
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Well, when I specifically asked the help for hidden gems, it said "No results for hidden gems. Please try again." An interesting tidbit is that if you search for "no results" you get lots of results! Curiously, it even suggests that you search for "no results," but none of the hits it brings back are related to no results (other than having the word no and the word result in there). If you search for gems (not hidden ones), you'll find one in base MATLAB, and one in the Image Processing Toolbox.
Of course Google found lots of hidden gems: Hidden Gems Near The Mathworks