Mostly gone !
For the VFR pilots the outer & middle markers are usually associated with an ILS system, the outer issued as a system check, as you get the audio and light you check that you are passing the correct altitude with the glide slope centered ( Go around if anything is not correct) the middle marker is almost always placed at the CAT 1 decision point and it should prompt you to see the approach lights or runway or go around.
history the inner marker was in the early days of ILS placed at the runway threshold, it those days there was no hard decision altitude and it was intended to help the pilot find the runway, this of course was found to be dangerouse to say the least and this practice was discontinued. Some inner markers could be found on airway centerlines, the last one I can remember was on Killiney hill just south of Dublin.
As said above DME has largely taken over from markers as the DME station can be securely placed on the airfield, Markers being placed about four miles from the threshold where very vulnerable to vandalism and theft of the wiring and electrical equipment and this is why at least at two airfields I know of they have been withdrawn in favoir of DME.
As said above Cranfield and Ostend still have markers and there are quite a few once you get outside Europe, there have been experiments using low powered voice transmissions on the marker frequency to warn aircraft on the ground that they are approaching the active runway holding point, this is intended only for CAT 3 operations.
I hope this has been helpfull !