Terragenesis offers a great photo tutorial for making medieval wargames buildings.
Realms of Chaos Blog has a posting about the old GW game called Chivalry, which appeared in an issue of White Dwarf. All the relevant pages there are posted, so you can recreate this classic on your own. It’s a card system for combat between knights.
The DMMapp (Digitized Medieval Manuscripts App) links to more than 300 libraries in the world. Each one of these contains medieval manuscripts that can be browsed for free.
Tournament Joust is a free print and play game with some really good looking jousting mechanics. It has paper knights, but you’ll want to swap in your own miniatures.
Games Workshop’s Lord of the Rings rules set has turned out to be very flexible, spawning several commercial sets, such as Legends of the Old West and Legends of the High Seas and innumerable fan variants. Legends of the Feudal Times is a set of feudal skirmish rules originally published in the French language magazine Vae Victis. The English translation is here.