From the fansite Westeros.org, A Wiki of Ice and Fire focuses mainly on the books, and has so many entries on so many pieces of George R. R. Martin's universe that you can fall down a rabbit hole and not emerge for days. If you're watching the show or reading the books and want a reminder or more information about pretty much anything, search AWOIAF and you'll learn anything you need.
This wiki focuses more on the television show than the books. From the home page, you can browse major houses, areas of the Known World, timelines and more. The site also features a section for recent news and updates.
Westeros.org - a vast collection of sites including fan sites, official sites, a forum, the Wiki of Ice and Fire and Blood of Dragons, an "on-line, text-based role-playing game, fully approved by George R. R. Martin."