This Wordorigins.org post by ᴚǝǝƶɐʍɐɈ is full of language sites I hadn’t known about, most of which I won’t use enough to put on the sidebar but which some of my readers may be glad to have on hand, so here are the links — you can read more about them at the linked post:
Lexikon zur byzantinischen Gräzität (I did add this one to the sidebar).
Tesoro della Lingua Italiana delle Origini (not yet complete).
Corpus Diacrónico del Español (just citations, no definitions).
AlWaraq.net (“does for medieval Arabic what CORDE does for medieval Spanish”).
Chronologisch woordenboek, which gives the date of first attestation of Dutch words, downloadable as a PDF here.