The armour archive is, well as the name suggests an archive for medieval armour patterns. In it you can find the designs for quite a lot of armour pieces, ranging from helms to breastplates, gauntlets and greaves. It has a good ammount of essays on the matter, covering a lot of the basics on finding the right tools and putting them to good use. Its fairly comprehensive and can be quite helpful since you can literally download and print out the patterns you want. On the other hand I don't think it has been updated in quite a while, not that there is much need for it.
The site also has a fair amount of links to other pages on the topic, however not all of them are working - the issue of updating is here as well.