Leader/follower, primary/replica, sender/receiver, primary/fallback... not only do these phrases avoid the charged language of "master/slave", but they can also often describe the roles of the two components more precisely.

The historical usage of "master/slave" for the two drives on an IDE channel is a great example of the imprecise use of these terms -- it implies a relationship between the devices that doesn't exist at all. Both devices respond directly to commands from the host; the second device on the channel is not under the command of the first one.