The stickers are completely open; Anyone can submit new packs. Animated stickers are also supported. The clients are all opensource, and the APIs usable by any

The stickers are completely open; Anyone can submit new packs. Animated stickers are also supported. The clients are all opensource, and the APIs usable by anyone. It has tons of little nice features as well. E.g., organizational directories, great search, fine grained notification settings, … . There is also voice chat support, which is pretty nice.