There are many digital ink options available to users; I have listed some options in the right-hand column.  Several of the annotation applications can be freely downloaded from the manufacturers’ website, and most will work on other touch-capable surfaces, i.e., the Sharp PenSoftware tools will work on a Wacom tablet.  Most of the third-party applications offer a trial period if you would like to first try-before-you-buy, including Ink2Go.