For now, this is a list of just a few of the sites out there about bharatnatyam and other forms of classical dance.
The Tehreema Mitha Dance Company is based in Washington D.C. and performs classical bharatnatyam and contemporary dance. I've seen Tehreema perform a number of times and she's been spectacular. If she and/or the dance company itself is performing anywhere near you, get thee to a ticket counter.
For an introduction to bharatnatyam itself, as well as other classical dance styles (odissi, kuchipudi, and others), take a look at Nandanam.
Medhahari's site is another good resource, particularly if you want a more in-depth look at the dance form.
Srividya's site is also a good source of detailed information on bharatnatyam.
I had the pleasure of watching Sharon Lowen give a lecture demonstration at Mazmoon-e-Shauq in 2006. She specializes in Odissi, Manipuri, and Chhau. The Odissi, in particular, was mesmerizing.