Why hasn’t someone done this before? As a geneticist, I’m constantly reverse-complementing nucleic acid sequences, but have been completely surprised nobody else has made a simple widget to do this quickly, rather than loading a large bioinformatics program.
For Mac OS X Dashboard.

This work is released under the Creative Commons License. You may use all parts of it for your own projects but MUST ATTRIBUTE THE ORIGINAL AUTHOR: Rich Kulesus (mrbiotech).