Several months ago I had an interesting project to create icons for a scientific computer desktop application called SAAMII Simulation Analysis and Modeling. My client was The Epsilon Group, a small company that makes pharmaceutical modeling systems. They were rewriting a Windows application for the Mac. I studied the Mac OS X icon guidelines, and delivered png files in 32px square format, and retina-display-ready 64px square format. Some of the icons were delivered as PDF files, because of their context in a toolbar.
Here are some examples of the icons I made:
Here is how the icons looked in the application:
There were a lot of icons in this project, as there are many actions for the user, and the icons are expected as ways to quickly remind a user about the actions of the program. I think they do the job.
I used Illustrator for the vector art work, but used Photoshop to generate the .png files, because it does a much better job of it than Illustrator.
I did a lot of online research into images, and reused bits of images found here and there. Working on designs that will perform well at very small sizes is an interesting challenge. I tried to make the images distinct and conceptual.