This guy has created some really nice character portraits as well as scene and location illustrations. There are also some maps (with greater detail than the published maps).
Another useful online encylopedia/reference. The 'Feast for Crows Primer' is particularly helpful if you've forgotten some of the key details owing to lengthy gap between ACoK and AFfC.
There are so many characters, places and history behind George Martin's 本 that I often forget who's who and what's what. The faqs, encyclopedia and concordances are indispensible for the enthusiast.