Science Online tables of contents are available in this format. The online presence of some journals consists only of subscription information or tables of contents with or without abstracts. Many journal publishers allow access to the full issue if you or your institution subscribes to the print version. In some cases, there is a registration process before full text can be accessed through the institutional subscription. Others allow free access to portions of the journal.
Some publishers allow purchase of online subscriptions without purchasing the print counterparts.
Yes, there are several electronic journals without print counterparts. There will most likely be a lot of change in this area as publishers try to find ways to make a profit in a new medium. Other players in the e-journal marketplace are vendors and aggregators who package groups of journals into bundles and who negotiate deals with libraries and consortiums. They are a non-profit organization and part of their mission is to educate or inform. They want to advertise.