GMT Games offers a free download of a Battle of Agincourt game. If you’re familiar with their commercial board war games, you’ll know that they have wonderful graphics and gameplay, both of which are on display here.
The Internet Medieval Sourcebook is a fantastic resource for miniature wargamers. I’ve had this one in my bookmarks for several years and haven’t even begun to plumb its depths. The section on the Crusades is particularly useful.