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.

How to Model Crests or Helmets
Whenever you have occasion to make a crest or helmet for a tourney, or for rulers who have to march in state, you must first get some white leather which is not dressed except with myrtle or ciefalonia, stretch it, and draw your crest the way you want it made. And draw two of them, and sew them together; but leave it open enough on one side so that you can put sand into it; and press it with a little stick until it is all quite full. When you have done this, put it in the sun for several days. When it is quite dry, take the sand out of it. Then take some of the regular size for gessoing, and size it two or three times. Then take some gesso grosso ground with size, and mix in some beaten tow, and get it stiff, like a batter; and put on this gesso, and rough it in, giving it any shape of man, or beast, or bird, which you may have to make, getting it as like as you can. This done, take some gesso grosso ground with size, liquid and flowing, on a brush, and you lay it three or four times over this crest with a brush. Then, when it is quite dry, scrape it and smooth it down, just as you do when you work on panel. Then, in the same way, as I showed you how to gesso with gesso sottile on panel, in that same way gesso this crest. When it is dry, scrape it and smooth it down; and then if it is necessary to make the eyes of glass, put them in with the gesso for modeling; do modeling if it is called for. Then, if it is to be gold or silver, lay some bole, just as on panel; and follow the same method in every detail, and the same for the painting, varnishing it in the usual way.

Cennino Cennini, Il Libro dell'Arte

• Fols. 65v and 66r, the Smithfield Decretals (Brit. Lib. Royal 10 E IV), c. 1300-1340
• The Manesse Codex (Cod. Pal. germ. 848), c. 1305-1340, features crests in virtually all of the illustrations. Those in which the crest is shown on a helmet that is worn (or held) by a person include: Herzog Heinrich von Breslau (fol. 11v), Der Herzog von Anhalt (fol. 17r), Herzog Johann von Brabant (fol. 18r), Graf Friedrich von Leiningen (fol. 26r), Graf Albrecht von Heigerloch (fol. 42r), Graf Wernher von Homberg (fol. 43v), Herr Walther von Klingen (fol. 52r), Herr Heinrich von Frauenberg (fol. 61v), Der Schenk von Limpurg (fol. 82v), Herr Hitbolt von Schwangau (fol. 146r), Herr Wolfram von Eschenbach (fol. 149v), Wachsmut von Künzingen (fol. 160v), Herr Walther von Metze (fol. 166v), Herr Hartmann von Aue (fol. 184v), Herr Goesli von Ehenheim (fol. 197v), Winli (fol. 231r), Herr Ulrich von Lichtenstein (fol. 237v), Der Püller (fol. 253v), Herr Dietmar der Setzer (fol. 321v) and Der Dürner (fol. 397v).
• Fols. 82r, 163r, 187r, and 202v of the Luttrell Psalter (Brit. Lib. Add. 42130), c. 1325-1335
• Crest of the Black Prince (side view), 14th century
• The Prankher helmet, with crest in the shape of buffalo (?) horns, c. 1350
• A buffalo-horn crest, which may have belonged to Ulrich IV. von Matsch, c. 1350-1375
• A tournament, Roman du Roy Meliadus de Leonnoys (Brit. Lib. Add. 12228, fols. 150v-151r), c. 1352
• Guiron le Courtois (BNF NAF 5243), c. 1370-1380: Brunor d'Estrangoire, Morhaut, Gawain and Blioberis (fol. 17r); Gawain, Blioberis, Kay, and Sagremor (fol. 19r); Meliadus leaves the knights (fol. 38r)
• The funeral achievements of Edward, the Black Prince, 1376
• Many in BNF Fr. 343 (a manuscript with several Arthurian stories), c. 1380-1385, including Departure for the Quest of the Holy Grail (fol. 8v), Vagan at the departure for the Quest (fol. 9), Punishment of Bademagu (fol. 10), Galahad (fol. 12), Battle of Camelot (fol. 62v)
• The Jousting-Hall frescoes (details here and here and here) at Runkelstein Castle at Bolzano, c. 1390-1395
• Allegory: Psychomachia, The Books of Modus and Ratio (BNF Fr. 1297, fol. 154v), first quarter of the 15th century
• UBH Cod. pal. germ. 359, c. 1418:
  The duel of Sigstapp and Reinholt (fol. 26v)
  The duel of Hagen von Tronege and Wolfhart von Garte (fol. 29v)
  The duel of Aspiran and Wittich (fol. 32r)
  The duel of the giant Schrutan and Heime (fol. 34r)
  The duel of Stueffinc and Dietleip von Stire (fol. 35v)
  The duel of Rüdiger and Gernot (fol. 39r)
  The duel of Walther von Kerlingen and Hartunc (fol. 40v)
  The duel of Volker and the monk Islan (fol. 46v)
  The duel of Dietrich and Siegfried (fol. 49r)
  The duel of Hildebrand and King Gibeche (fol. 57v)
• Heraldic initial, Froissart's Chronicles (Brit. Lib. Arundel 67, vol. 2, fol. 1), 15th century
• The king of Portugal dressed for a joust, c. 1433
• Tournament for the arrival of Isabeau of Bavaria at Paris, Froissart's Chronicles (BNF Fr. 2648), mid-15th century
• Joust at London, Froissart's Chronicles (BNF Fr. 2646, fol. 92v), second half of the 15th century
• Several examples in the Tournament Book of King René d'Anjou (BNF Fr. 2695, c. 1460; BNF Fr. 2692, c. 1488-1489; and BNF Fr. 2693, c. 1488-1489). Most notable are the illustrations demonstrating the assembly of the crest on the helmet (BNF Fr. 2695, fol. 23v and BNF Fr. 2692, fol. 20 and BNF Fr. 2693, fol. 14) and the helmshau (Fr. 2695, fols. 67v-68 and Fr. 2692, fols. 47v-48 and BNF Fr. 2693, fols. 41v-42).
• Sir John Astley jousting (Justus of the Pees; A helme well stuffyd with a crest of hys de vijs), Ordonances of Chivalry (PML M.775, fol. 2v), c. 1470
• Tournament at St. Inglevert (fol. 43r) and riding to a tournament in London (fol. 99r), Froissart's Chronicles (Brit. Lib. Harley 4379), c. 1470-1475
• Tapestry of a tournament (detail), end of the 15th century
• Three crests, c. 1500: a crown, a tower, and a bearded man (side view), compare to the grave-stone of Wolff Conrad von Poessnitz und Weittersfelden at Weiner Neustadt, 1574); a bearded man on a crown, between two horns; a lion on a crown
• Detail from The Bearing of the Cross by Jörg Breu the Elder, 1501
• Tournament-book of Frederick III and Maximilian I (BSB Cod.icon. 398), c. 1489-1511
• Olivier de la Chapelle, d. 1508
• Köldererrolle, c. 1512-1513
  (This seems to be a roll of arms, so most of the armor/costumes are really more symbolic than literal.)
• St. George Freeing the Princess by Georg Lemberger, 1520
• June, The Golf Book (Brit. Lib. Add. 24098, fol. 23v), c. 1520-1530
• February, Die Augsburger Monatsbilder, 1520s
• Portrait of Francesco Maria della Rovere, Duke of Urbino by Titian, 1536-1538
• Burgkmair Tournament Book (BSB Cod.icon. 403), c. 1540
• Portrait of Maurice, Elector of Saxony and another portrait of Maurice, Elector of Saxony by Lucas Cranach the Younger
• Emperor Charles V at Mühlberg by Titian, 1547
• Portrait of a gentleman by Giovanni Battista Moroni, c. 1550
• Album of tournaments and parades in Nuremburg (Met 22.229), recording a series of jousts held in Nuremburg between 1446 and 1561; see also The Metropolitan Museum Journal v. 45
• Gravestone of Arnould de Berlo and his wife Marie de Coutreau, c. 1561
• Portrait of Alof de Wignacourt by Caravaggio, 1607-1608