I love the Norton Anthologies of Literature. I have been a Norton fan since my undergrad days when we used them in my survey classes, and now that I'm teaching lit surveys I'm passing these wonderful collections along to my students (by which I mean forcing my students to buy them).
The Norton editors are really good about providing background for such a wide variety of texts, including general introductory material about the medieval period--social organization, religious developments, changes in the English language from Anglo-Saxon Old English, through Norman French to Middle English, and into Early Modern English. The editors also provide more specific background for the individual texts, giving some history of the author (when it's known) and the significance of the text in the overall picture of English literature.
For my British Literature I survey, I selected a variety of texts, including "Cuchullain's Boyhood Deeds," the opening section of Beowulf, pieces of Chaucer's "General Prologue" and "Wife of Bath's Prologue," the "York Play of the Crucifixion," and a couple of pieces from Margery Kempe. As this list suggests, the Norton has a wide range of texts (and I didn't even cast as wide a net as I initially intended to), dealing with a multitude of issues that were important to medieval people, including religion, gender roles, chivalry, the shift from Anglo-Saxon/Old English to Norman/Middle English, and so on.