It has been in the protocol since 2010 as I can see but, yet no one really has implemented that into interface, is that right?
Blockchain seems to have had it for a while but then removed it
Multibit wallet is working on it but we don't know when it will be available
so we can only use it by command line with the original bitcoin client?

is there anyone else that i'm missing and has it?

and how come such significant feature is so under-used?