I used XaraX to make the original set. It’s not free, but it’s not expensive either. You can download the *.xar files from here: http://www.grasshopper3d.com/forum/topics/grasshopper-icons-vector-sourc That page also explains the post-process.
You can however make icons through code as well, it’s just bitmaps. If your icons are made up of a boundary shape, a colour and a text label, just use system.drawing to generate them at runtime.
ps. I no longer have XaraX installed these days, I’ve lost the CD and can’t be bothered to get a new license. On those rare few occasions I have to make icons these days I pixel edit them manually in Paint.NET