Medieval and Renaissance helmet-crests, including extant examples, with some from illustrations (though moreso from people wearing or holding a helmet with a crest, rather than the crest as a part of a symbolic or decorative heraldic achievement).

See also A casual discussion on making Medieval crests for tournament helmets, Fox Tournament Helmet Crest, and Building Crests for Mounted Combat.

I've included several examples of plumed helmets in this linkspage as well.