The Middle Ages

Era of the crusades, Robin Hood, and fat, lecherous (but good hearted) friars. Also home to dragons, wizards, knights in shining armour, beautiful princesses with big headdresses and tall, spirally gothic architecture. Expect to see a Corrupt Churchman or two wandering the landscape burning witches, heretics, and pretty much anyone who doesn't agree with them. Also expect to see people comically dropping left and right from the Black Death. ("Bring out your deeeeaaaaa----" *Fall* *Splut!*)

If the story does not take place in some unspecified kingdom, the setting is likely to be a mythologized Britain or France, though Jerusalem, which had pretty much been ignored since Biblical Times will now also be used as the stage for all sorts of flashy battles involving cutlass-wielding Turks and ruggedly handsome Knights Templar.

For the (slightly) more historically accurate version of this trope, check out its constituent periods:

Cf. Ye Goode Olde Days for the highly romanticized version, and The Dung Ages for the depressingly bleak version. When history has been abandoned altogether, you have probably made the epic voyage to The Time of Myths.

The Middle Ages provides examples of the following tropes:

Black Knight
Christianity Is Catholic: Though it would eventually be throughout most of Central and Western Europe, in Eastern Europe and the Near East, Orthodoxy was more prevalent; yet is much less common in fictional works. (And officially, the final split came in 1054, and reunification was seen as a viable possibility until 1204.)
Courtly Love
The Crusades
The Dung Ages
Ermine Cape Effect
Everything's Better with Princesses: There weren't really many more then than now.
- Except a lot of small kingdoms have been eaten up since then and a number of states have changed from kingdoms to republics, so there were a lot more reigning princesses. There may be girls today who, if you assumed the continuance of feudal law, would in fact qualify as princesses, but who work as waitresses and don't know they are titular princesses.
Feudal Overlord
Gorgeous Period Dress: Except for (ugh!) Ye Peasants and one or two of the more self-denying Churchmen. Look for puffy, slashed sleeves and trunk-hose among the men, though these were really Renaissance fashions.
Historical Domain Character: Even when they happen to be talking lions.
Horny Vikings
Knight in Shining Armor
The Knights Hospitallers
The Knights Templar: The original Knight Templar characters came from this era.
Medieval Morons
Nice Hat: When the tall headdresses, including the famous steeple headdress (or hennin), were in vogue.
Pimped-Out Dress: Lots of fur, silk, and velvet for the nobility (though the last was not actually invented until very late in the period).
The Plague
Swashbuckler: One of the two classic periods for the setting.
Vestigial Empire: The Roman Empire was still around, based in Constantinople. In fact, historians often define the Middle Ages as the period from the fall of the Roman Empire in the west (476) to the fall of the Roman Empire in the east (1453). Restoring The Empire is a popular scenario, especially in Strategy Games.
- It also should be noted that it took several centuries after the fall of the Roman Empire for the general population to accept that the Empire was really gone. This applies to both the eastern and western empires.
Ye Goode Olde Days
Ye Olde Butcherede Englishe: Though actually rarer than one might expect in most serious modern works, 'tis more common in older ones. ("'˜By the light of Heaven!' said Prince John to Hubert, 'an thou suffer that runagate knave to overcome thee, thou art worthy of the gallows!'")

Works set in this time period include

Comic Books

Demon Knights
Prince Valiant

Film

Army of Darkness
Braveheart
The Court Jester (Stated to be in the reign of King Roderick I. of England.)
Dragonheart
Dragonslayer
First Knight
Ladyhawke
All permutations of Robin Hood.

Literature

Doomsday Book
Michel Pagel Le roi d'août (The King of August), a novelized version of the life of Philip II Augustus with fantastical elements (dryads especially). Quite historically accurate (except for the obvious Fantasy).
Wolfbreed takes place during this time period and deals with The Teutonic Knights' conquest of Prussia...and werewolves.
Catherine, Called Birdy and The Midwife's Apprentice, both by Karen Cushman.

Live-Action TV

The first Blackadder series.
Beverly Hills 902 A.D.
Most TV and movie adaptations of King Arthur, despite the "correct" setting being immediately post-Roman Britain. This is because the earliest written versions of the legend were written in the Middle Ages and given a "contemporary" makeover.
The Mystic Knights of Tir Na Nog
Most TV and movie incarnations of Robin Hood.