Germanic languages
For those doing research in older Germanic languages (Gothic, Old Icelandic/Norse, Middle/Old High German, Frisian, Old English, etc.), Sean Crist’s Germanic Lexicon Project is a great resource for old grammars and primers and dictionaries. He’s scanned several out-of-copyright books and put the images online, with a portion of those OCRed and/or in a nice text/HTML/XML format. My own digitization of Henry Sweet’s An Icelandic Primer is on the site as well (that’s where I originally got the images). I’m considering digitizing an early edition of Sweet’s Anglo-Saxon Primer but haven’t made up my mind yet.