KSEE in San Diego CA. has a FM on the LOC approach. The intro section of the Jepp plates talk a little about it, the AIM doesn't have anything too good on the topic either. To my understanding they are a less precise marker beacon which has a larger signal area. Used for non precision approaches, mainly back course approaches to identify a fix, and I think DME has pretty much taken it's place. Email your FSDO and you should get an exact answer within a few days, they are pretty good about that.