DogNZB (Invite-Only)

This site has been around for a long time and has a great custom frontend - the push capabilities are most noteworthy.

OZnzb (Invite-Only)

OZnzb was once one of the big public nzb sites but closed its registrations after a prolonged downtime.

nzbhydra (Open Registration)

nzbhydra is open for registration and has a nice and modern user interface. Very fast response times to.