The Rule of Tincture

Heraldry of the World - The largest online database of heraldry.

Héraldique Européenne - Arms of a tremendous number of royals and nobles, past and present, from all over Europe. Includes rolls of arms for the major knightly orders. It's in French, but pretty navigable for the non-speaker.

A Glossary of Terms Used in Heraldry - A digitization of an 1894 book by James Parker. Very comprehensive. If you can dream it, chances are Parker's got a blazon term for it.

Basic Blazonry - A primer for beginner blazoneurs from the Society for Creative Anachronism.

Heraldry for Scribes - Another SCA guide. It also has tips on designing and drawing a good shield.

A Grammar of Blazonry - SCA again. The advanced course, for people who have mastered basic blazon writing. The difference between “palewise in fess” and “fesswise in pale,” things of that sort.

An Armory of Famous Musicians - Not too useful in the AH area, but it's really interesting stuff and includes good examples of complicated blazons.

Mon - A basic introduction to Japanese heraldry

Kamon World - This used to be easier to navigate without knowledge of Japanese. Now I have to click around randomly. But the kamon collection is just as enormous and just as beautiful and just as useful for people looking to design their own.

International Heraldry & Heralds - Heraldry in various countries, including the right to arms and armory for genealogists. Extremely extensive research.

Wappenwiki.org - A great MediaWiki project that collects historical coats of arms from all over Europe (sorted by geographic and historical regions).