The quantity of material online is getting truly overwhelming. With the exception of modern scholarship, which is still largely confined to print, I do believe there is enough material online for the teaching of any undergraduate history course. At least for European (and probably for U.S.). Really, for many areas, there's even enough to support graduate research, for an enormous number of archival collections are also coming online. Most are free, though some do require a subscription.
The latest impressive resource I've found is at the Internet Archive. A search in their text collection yields an extraordinary treasure trove. True, it's mostly very old, as it must be out of copyright protection, but in some ways this is an important supplement, for these are the very works that tend not to appear on most library shelves.
Now, granted, an 1850 history of medieval Germany is not where I would have most students begin. On the other hand, though, a search on "Reformation" turned up collections of source documents that included a collection of letters sent from English reformers to people like Heinrich Bullinger. You simply are not going to find those letters anywhere except in a specialist library, and then you may not be able to get the work via interlibrary loan, depending on its condition. Because it's online, though, I can make assignments into it for my students. These are voices my students would otherwise never hear.
I encourage my peers to explore these resources. It does take a tremendous amount of time. Perhaps there's work to be done here, compiling references to specific online works (even down to particular pages or passages), in the same way paid scholars once collected materials for a printed collection of teaching resources.