About.com: Panic/Anxiety Disorders
http://panicdisorder.about.com/
Like most About.com topic sites, this one has a decent assortment of informational articles and a lot of pointers to other resources on the web. A good starting point.
Anxiety Disorders Association of America
http://www.adaa.org/
Web site of a large mainstream nonprofit organization for people with anxiety disorders. Includes information resources (with special sections for women, teenagers and kids), information on self-help and finding a therapist or support group, a newsletter, conference information, and more.
AnxietyPanic.com
http://www.anxietypanic.com/
I have mixed feelings about whether to include this site or not. At first glance it looks nicely designed and information-rich, but a closer looks shows that a large proportion of what look like article links are basically ads for different treatments of varying degrees of dubiousness. However, in between all the infomercial-type stuff there is some actual information on the disorders, book reviews, news items, etc. So I guess I'd say go ahead and visit, but read critically.
Freedom from Fear
http://www.freedomfromfear.org/
Web site of a US organization for people with panic and anxiety disorders. Contains a lot of information articles, discussion forums, information for health professionals, a directory of support groups within the US, and more.
National Panic and Anxiety Disorder News
http://www.npadnews.com/
Another anxiety/panic disorder site. Kind of ad-heavy and not terribly well designed, but there's at least some useful information here.