Inkscape
I’m a big Adobe Illustrator user. But I realize that is a rather expensive program for the average mac user to have to penny up for. So I went in search of an application that could fill in for those who don’t have the resources. I came across a very capable program called Inkscape. It requires X11 to run. It is cross platform and runs in Linux, Windows and Mac. From my limited experience with it, the application has most of what you need to build web images. It even exports in .png.
As you can see I’ve drawn two boxes. One white one with a black stroke on top of a blue box. I’ve applied transparency to the top box. The fill and stroke properties are open in the palette to the right of the main drawing board. If you are used to mac keyboard shortcuts, you have to remember to use the control key instead of command, as the X11 application is Unix oriented.