There are so many websites promoting veganism that it would be impossible to name them all.
Instead here is a small list of sites that I find interesting.

Vegan Police  http://theveganpolice.com

WE ANIMALS  http://www.weanimals.org

Mercy for Animals  http://www.mercyforanimals.org

SHAC (Stop Huntingdon Animal Cruelty)  www.shac.net

Green is the New Red  http://www.greenisthenewred.com

Sea Shepherd  http://www.seashepherd.org

Maximum Tolerated Dose (documentary)  http://maximumtolerateddose.org

Physicians committee for responsible medicine  http://www.pcrm.org

British Union for the abolition of vivisection  http://www.buav.org

The Veganomaly  http://theveganomaly.com

Farm Sanctuary  http://www.farmsanctuary.org/