The answer to your question is any business that there is a hungry market for. But one where you don't have to physically store products in your garage and then have to ship them out.
This is why selling digital products or services are the easiest to do.
My own business is selling video spokesperson to businesses with a website. I advise them that I can enhance their website by supplying them with the html code to place a video spokesperson, (which I easily create from software) on their site.
The beauty of this service is that you can charge whatever the market will bear. Usually anything up to $1000.00 per client. Any business, whatever it is, as long as they have a website will love this.
If you want to take a look and see how easy it is to do, check it out here.