GIPHY for iMessage - The easiest way to share all the GIFs in all your txts
Stickeroid - Rate Everything
MojiLaLa Unlimited for iMessage - 32,000 original stickers by 450+ artists all over the world.
Sticker.Place API - Get stickers for your app
Doll Up Stickers - Dress up your messages.
Sticker List - Discover the best stickers daily and publish your own.