ZooKeeper is really the most accepted option here. It is used by many other important projects (Kafka, Storm, Hadoop) so adding it to your stack is likely to pay dividends later when you want to adopt some other cool technology that relies on it.

Doozer is the other option I've heard of that fills this role, but I don't personally know of anyone using it.

In terms of initial discoverability, services register in a designated place in ZooKeeper using an ephemeral node and then other services can find them there. You can set a "watch" on the ephemeral node to get notified if it goes away for any reason (ie. the other service or node goes down)

Doozer: https://github.com/ha/doozerd