Aberdeen Bestiary (original page; for an easier access see this link
Medieval Bestiary (including the weasel)
Codex Gigas (Vulgate, 12th c., Bohemia)
French medieval mss., plus a huge bibliography
Illuminated German manuscripts, catalogue, with search functions
Manuscripts in libraries: great selection
Dresden: Werner J. Hoffmann, Die Mittelalterlichen Deutschen und Niederländischen Handschriften der Sächsischen Landesbibliothek – Staats- und Universitätsbibliothek Dresden, (Archivum Medii Aevi Digitale – Studies: Catalogues 1), Frankfurt am Main 2022, ISBN 978-3-95908-535-9, DOI: https://doi.org/10.25716/amad-85248.
Marginalia – with grotesque examples
Marginalia – in light of Covid-19, fun and instructive
Bunnies in medieval manuscripts
Wasserzeichen:
http://www.ksbm.oeaw.ac.at/wz/wzma.php
Marburger Repertorium (Mittelalter)
Marburger Repertorium (Uebersetzungsliteratur, Humanismus)
Die Hausbucher der Nurnberger Zwolfbruederstiftung
Swiss mss., or in German Swiss mss. 2 (facsimiles)
Austrian medieval manuscripts I and II (more comprehensive)
Global Catalogue of Digitized Manuscripts
Oxford Catalogue of Incunabula (Bodleian)
Prachteinbaende. Muenchen. Stbibl.
Illustrated Blockbooks (up to 1500) in the StaBib. Munich
Manuscript Finder (commercial, but provides a quick look at many different facsimiles)