Gravity X 3
Newton's theory of gravity is a darn good theory. If you want to calculate the trajectory of a projectile or the orbital path of an interplanetary vehicle, Newton's your man. Ditto if you want to calculate the pressure at the center of the Sun. Of course your answers might be ever so slightly off - which is where...
Einstein's theory gravity, AKA General Relativity, comes in. It's a bit unwieldy with a whole potfull of nonlinear partial differential equations, but it can fix up those orbits. It can even let you calculate the pressure at the center of a white dwarf or a neutron star. It also tells you how to fix up your clock times in the presence of strong gravitational fields. What it can't do is tell you what's happening at super strong fields at the Planck length, or at the center of a black hole.
Really good theories make nice testable predictions, usually with important practical consequences, like keeping your GPS satellites synchronized. String Theory is not that good yet. We can't test its gravitational implications, and, so far as I can tell, its descriptions of real black holes are still incomplete. So what can it do? Well there is one other virtue a good theory can have, and that's to serve as a fruitful source of ideas for other areas of physics. Sean Carroll says:
One of the reasons why string theory is so popular among people who have thought about it very carefully is that it really does lead to new things. It really is fruitful. It's not that you have make some guess like, oh, maybe space time is discrete or maybe the universe is made of little molecules or something like that, and then you say, okay, what do you get from that? By making this guess that instead of particles there are little strings, you are led to thinking if I put that into the framework of quantum mechanics I get 10 dimensions. Then, oh, it also needs to be supersymmetric. There are different kinds of particles that we actually observe in nature and if we try to compactify those extra dimensions and hide them, we begin to get things that look like the standard model. We are learning things that make us think that we are on the right track.
Sean has a lot of other stuff in his article, including a discussion of a question that has often vexed us - the entropy of the early universe, and why it's important. I recommend the link.