This site has excellent references to common interventions as well as an interactive birth plan. The motto of this site is "Birth is a natural process, not a medical procedure".
The Bradley Method Homepage
I listed this not only to introduce you to The Bradley Method, but for its link on the Pregnancy Diet. The protein counter is especially useful.
Lamaze International Homepage
This site contains info on how to find a class in your area, books for expectant parents, and the basic principles of the Lamaze method of natural childbirth. Pregnancy & Childbirth
Very inclusive site with information on everything from conception to baby care.