In my Tools of the Trade posts I often highlight kitchen tools that I find very useful, but not essential.  A knife sharpener, however, is essential for me.  A lot of cutting goes on in the kitchen, and that task is so much easier and in some ways safer with a sharp knife. 

I have not invested in an expensive knife set.  I understand the importance of good quality tools, but high quality always comes with a high price.  Instead, I have decent knives that work well when they are sharp.  But they do get dull. 

I have a diamond hone sharpener like the one in the picture ($30.90 from Amazon), but there are others to choose from