relational quantum mechanics
Relational quantum mechanics, proposed by Carlo Rovelli in 1996, makes a single sweeping move: it denies that systems have states by themselves. A quantum state, and the values of physical quantities, are always relative to some other system that interacts with the first — much as velocity in relativity is never absolute but only velocity with respect to a chosen frame. There is no view from nowhere, no observer-independent set of facts about which outcome occurred.
On this reading a measurement is just an ordinary interaction in which one system records a definite value of another. That value is real for the recording system, yet it need not be settled for a third system that has not yet interacted with either. The Wigner's-friend puzzle, where a friend inside a lab sees a result while an outside observer still describes the lab as a superposition, stops being a contradiction: both descriptions are correct, each relative to its own system.
The appeal is a thoroughgoing democracy among physical systems — no special observers, no magic measuring devices, just interactions all the way down. The honest difficulty is making sense of how the separate, relative facts hang together into the shared, stable world we seem to live in. Defenders address this through the consistency of cross-records, but how completely that recovers ordinary objectivity remains debated.
Like velocity in relativity, quantum facts are always with-respect-to some other system.
Relational quantum mechanics is not the claim that reality is merely a matter of opinion; the relative facts are objective once you fix which two systems are involved. What it gives up is a single global list of facts shared by everyone at once.