The tremsetter's been made by a couple of companies over the years, currently Hipshot.
It's a simple but brilliant idea. It replaces the centre tremolo spring with one that compresses rather than stretches. The tension on it is adjustable, and once set up it will return a floating vintage tremolo to tune very well. At least, it does on my modified MIM with two springs in the cavity plue the tremsetter and a set of Fender 150 0.010’s. The centre of the trem travel feels like there's a detente there, or at least a bit like a pot with a centre detente.
It does seem to need the trem arm moving slightly more than a "normal" setup before things start happening and does alter the feel of the trem a bit. But personally I wouldn't be without one, it makes a bridge floated with the back of the bridge high enough to give a semtitone or a bit more up-bend stable, dampens out "trem flutter" issues when you hit the strings hard. String bending is also easier because the bridge is less likely to move so you get less of the trem bridge problem that while your fingers bend the note upward the string is also pulling on the bridge plate which rises, flattens the note you're playing and fights you.
They're easy enough to fit. You do need to drill a narrow hole in the body from the trem cavity towards the neck to accomodate a long thin rod that runs through the tremsetter body, but it's no big deal if you can drill reasonably straight. After that it's just a matter of adjusting it to get the feel and bridge float you want. I've found it helps on a six-screw vintage style bridge if the screws, especially the central four, are backed off a bit as well so they steady the bridge some but don't prevent it moving easily.
It's a product Fender ought to offer as standard in my opinion.