If you keep tabs on Android Wear, you've probably heard of Facer. It's the leading app for making and sharing custom watch faces for Android Wear. The lack of an official API has not deterred developers, and Facer isn't the only app offering new watch faces. Face for Wear has been in beta for a while now, but it's finally ready for everyone to check out.
Face seems to be going for a more controlled implementation of Android Wear faces than Facer, which is a bit like the wild west with random ZIP archives and Dropbox links being thrown around. Face relies on "premium" and user submitted watch faces that are distributed through the Play Store. This raises the barrier to entry, but it also allows designers to monetize their watch faces. Indeed, there are already a few that cost a buck or two. Some of the designs available at launch are pretty nice too.
There are still a few features missing in Face, and it is not yet clear if there will be an easy way to make and share your faces outside the Play Store. There is currently a rudimentary watch face designer in the app, but I don't know how far you'd get with it. At any rate, it's something to keep an eye on.