Westlake Village Aikido logo by Scott Cattanach
Website generously hosted by WebsiteASP
Aikiweb is one of the best Aikido information sites available. It also includes a very convenient dojo finder.
Even more information about Aikido can be found at Aikido FAQ which is not so much a FAQ as a collection of information.
Aikido Journal started as the best Aikido magazine in print. It has since abandoned its print roots, and become a strictly online magazine. It's still one of the best places to go to purchase Aikido materials (books, etc.), it has some interesting bulliten boards, and it's an all around font of Aikido information.
No Nonsense Self-Defense is a site run by Marc "Animal" MacYoung. Marc "Animal" MacYoung is one of the most knowledgeable martial artists in the world. This site is an amazing martial arts/awareness/fighting resource. You can learn more from this website than you can from most martial arts classes. Marc "Animal" MacYoung is also the author of many valuable self-defense books which can be purchased from most major online bookstores. I highly recommend all of them.
Iwama Shin-shin Aiki-shuren-kai (Hitohiro Saito - Iwama, Japan)
Patricia Yarrow's Saito Sensei Tribute Site
Aikido Iwama Ryu Europe lists many Iwama Ryu websites for dojos located throughout Europe.
Isoyama American Aikido Academy