So I know this is a rapidly changing field but (as of 2015-09-08) I found the following in the Docker Registry HTTP API V2:
Listing Repositories (link)
GET /v2/_catalog
Listing Image Tags (link)
GET /v2//tags/list
Based on that the following worked for me on a local registry (registry:2 IMAGE ID 1e847b14150e365a95d76a9cc6b71cd67ca89905e3a0400fa44381ecf00890e1 created on 2015-08-25T07:55:17.072):
$ curl -X GET http://localhost:5000/v2/_catalog
{"repositories":["ubuntu"]}
$ curl -X GET http://localhost:5000/v2/ubuntu/tags/list
{"name":"ubuntu","tags":["latest"]}