Seems that there is a bit of interest in marginalia at the moment.
Life on the Edge: Marginalia
A nice post on the John Rylands Library blog by John Hodgson with helpful images illustrating various cadels, drolleries, grotesques, and such things in mostly medieval manuscripts.
Marginalia - Scribes say the darndest things
Chris Brady picks out some nice photos from M. Camille, Image on the Edge: The Margins of Medieval Art (2004).
In other news I noticed this interesting discussion of the British Library's conservation of the fourth volume of Codex Alexandrinus (i.e. the NT) prior to digital photography (note the interesting photograph).