The Guide to Free D&D should cover all of them, since Wizards didn’t charge for the non-book options. Note that the list makes no attempt to separate things that are also available in some book from the things that are exclusively found online. Still, since it covers everything (probably, though extremely obscure offerings like the subject of this question may well be missed), with enough effort you can comb through it to find the ones you were unaware of.
Dragon and Dungeon magazines are the other major possible examples, though I’m sure you’re aware of them and they were published under license by Paizo, not by Wizards of the Coast.
The only other thing to be aware of is the various campaign settings that Wizards licensed off to other companies, which have a kind of nebulous “official-dom.” Most of those were published in books—though not by Wizards of the Coast—but athas.org, which is the “official” 3.5e version of the Dark Sun setting, never offered its PDFs in print to the best of my knowledge. From what I understand, one of the contributors to athas.org was a Wizards of the Coast employee—that’s partially how they got the “official” status—but the PDFs there aren’t Wizards of the Coast products, and aside from that one employee, have not been written, edited, or endorsed by the Wizards of the Coast staff. (To editorialize a little, their quality is also not nearly up to the standards of most Wizards products, though of course stand-out examples of terrible quality can also be found in Wizards’ catalog.)