Organizations:

Local / Las Vegas Area:

Las Vegas Freethough Society: http://www.lvfs.org
Las Vegas Atheists Meetup: http://www.atheists.meetup.com/30/?gj=sj5
Halvason: http://www.halvason.org

National:

American Atheists: http://www.atheists.org
Atheist Alliance: http://www.atheistalliance.org
Atheists Anonymous: http://www.geocities.com/atheist_anon
Atheists United: http://www.atheistsunited.org
Center for Inquiry - On Campus: http://www.campusfreethought.org
Council for Secular Humanism: http://www.secularhumanism.org
Infidels.org: http://www.infidels.org
Military Association of Atheists and Freethinkers: http://www.maaf.info
Positive Atheism: http://www.positiveatheism.org
Secular Coalition for America: http://www.secular.org

Information:

Infidels.org: http://www.infidels.org
Web of Reason: http://www.webofreason.org

Satirical:

Church of the Flying Spaghetti Monster: http://www.venganza.org
Landover Baptist: http://www.landoverbaptist.org

Activism:

American Civil Liberties Union (ACLU): http://www.aclu.org
Americans United: http://www.au.org
Freedom From Religion Foundation (FFRF): http://www.ffrf.org
Godless Americans Political Action: http://www.godlessamericans.org
God off Money: http://www.godoffmoney.com
People for the American Way: http://www.pfaw.org
Restore the Pledge: http://www.restorethepledge.com

Communities:

Atheist Meetup (networking groups): http://www.atheists.meetup.com
Atheist Network: http://www.atheistnetwork.com
Atheist Parents: http://www.atheistparents.org
Camp Quest (atheist summer camp): http://www.camp-quest.org
Freethinkers Match Maker: http://www.freethinkermatch.com
Freethinkers Personals: http://www.freethinkerpersonals.com
Secularity (secular social / dating service): http://www.secularity.com

Government Offices:

The White House: http://www.whitehouse.gov/contact
US Senate: http://www.senate.gov/
House of Representatives: http://www.house.gov

Shop:

Ring of Fire: http://www.rof.com
Landover Baptist: http://www.cafepress.com/landoverbaptist/