I’m a member of all sites
Care2 is great because not only can you sign petitions, you can read news stories, join groups etc. It’s like an ethical social network site.

These are all great sites and suggestions; thanks for sharing!

If you ever feel as though you’ve run out of things to do for PETA for the time being, you can always sign petitions to help animals. Once you fill out info the first time you just have to click sign and it will automatically fill out for you.
http://www.thepetitionsite.com/animal-welfare/activist/
http://www.gopetition.com/petition-campaigns/Animal-welfare/
http://www.care2.com/
There are many other sites, these are just examples. Enjoy!