M-theory
In 1995 Edward Witten started what has been called the Second Superstring Revolution by introducing M-theory to the world. This theory combines the 5 different string theories (along with a previously abandoned attempt to unify General Relativity and Quantum Mechanics called 11D-Supergravity) into one theory. What Witten actually did was to predict that the fact that all these different theories were connected was a result of there being some underlying theory of  which they were all approximations. This theory is somewhat vague in nature and has not yet been pinned down.