I allways prided myself for knowing (at least vaguely enough to be able to google it when needed) every piece of D&D 3.5 official content but today by a random chance I found something called Mind’s Eye: http://archive.wizards.com/default.asp?x=dnd/arch/psi
…which seems to be official content but never printed or released as part of any book.
I have no idea how I missed that but now I wonder whether I’m missing more. Were there any other official content (official as in released by WotC) that only exist in some article? Also if there indeed is more, why wasn’t it simply released as another book (not that I complain, free content is good, I’m simple surprised)?