The following is a list of some major church denominations and the websites for them
Seventh-day Adventist Church http://www.adventist.org
Assemblies of God http://www.ag.org
Southern Baptist Convention http://www.sbc.net
Catholic Church http://w2.vatican.va/content/vatican/en.html
United Church of Christ http://www.ucc.org
Churches of Christ http://church-of-christ.org
Episcopal Church USA (Anglican) http://www.episcopalchurch.org
Greek Orthodox Archdiocese of America http://www.goarch.org
Evangelical Lutheran Church in America http://www.elca.org
Lutheran Church–Missouri Synod http://www.lcms.org
United Methodist Church http://www.umc.org
Presbyterian Church–U.S.A. http://www.pcusa.org
Other websites for the major traditions
Calvinism http://www.reformed.org
Catholicism http://www.catholic.net
Eastern Orthodox http://www.assemblyofbishops.org
Orthodoxy http://www.orthodox.org
Mennonites http://www.mennolink.org