New York Aikikai http://nyaikikai.com/
This site is affiliated with Yamada Sensei, and the United States Aikido Federation.
This site is affiliated with Hombu Dojo in Japan.
This one's really nice. It has a great deal of information to browse through. This is one of the best Aikido sites I've found.
A Book of Five Rings http://www.samurai.com/5rings/
A worthwhile book to read, as it is written by Japan's legendary swordsman.
The Aikido FAQ http://www.aikidofaq.com/
This site has a lot of basic and other information. Video clips, too!
Body, Mind, and Modem http://www.bodymindandmodem.com
I highly recommend this site if you have Shockwave. There's even a neat samurai game... check this one out.