1,2,4,5 and 6 have come out on PS1 and GBA. 4,5 and 6 were basically just ports from the original SNES games with a few things touched up, but 1 and 2 were redone with SNES style graphics. The GBA versions have a few extra things in them, like extra dungeons. These versions of 1 and 2 were also on the wonderswan colour (Japanese handheld, can't get it anywhere else) and now they have also been released as mobile phone games. And of course the FF3 remake. That's the only one that's been a proper remake, although I suppose you could count 1 and 2.
Oh and I'll also add that Final Fantasy Tactics is coming to PSP. That means it'll finally get a release in this country (and even if it doesn't importing is no hassle since it's region free).