One of the most popular products to sell online is electronic books, or ebooks as they are commonly called.
Ebooks combine the best of the online experience by providing information (which is what most internet users are looking for) with the ease of a downloadable product. That means no inventory, packing or shipping. Also you capitalize on the internet users' demand for instant gratification. Wanting the information and being able to access it immediately are big draws for your customers.
If you're not selling the book, you can still use your content to create a free give-away for your subscribers. Not only do they get a valuable item for free (which is what everybody wants) but you can keep in contact with them and build on that relationship for other business offers.