goeasy,
Most modern airliners have one or two installed, that are triggered by a collision, or switched on manually. Each slide/raft also has one in the case of ditching.
I fly on the 747, 777, 767 and 787, of which none have an ELT per slide/raft in case of ditching. It is part of the equipment removal to be performed by the cabin crew, to remove the one, or two, portable ELT devices and re-locate them to the slide/raft in the case of a survived and evacuated ditching.