Just about every physics and math department at the major universities in the world have at least a few people working on this, often in groups studying pieces of the problem. Read Dr. Michael Kaku's article on string theory for some details. The main players I keep hearing about are Edward Witten, David Gross, J. Harvey, E. Martinez, R. Roth at Princeton; Steven Weinberg at U. texas at Austin; Andrew Strominger at UC Santa Barbara; John Schwartz and Michael Green the original founders os string theory.
There are literally HUNDREDS of people, graduate students and post-docs working on the 'Theory of Everything' producing hundreds of articles on various technical aspects of it, every year.
As a complete outsider, it seems they are certainly hot on the trail of cleaning up many annoying problems and insufficiencies with the theory, and just plain discovering how incredibly RICH a mathematical subject this is. The unification of gravity with the theory seems to be old-hat, but the most vexing problems still have to do with understanding why the energy of the vacuum is so low...if not non-existent...and to do so in a way that is 'natural' rather than ad hoc. The latest excitement comes from the study of quantum black holes and superstrings. It now seems to be possible to exactly calculate the entropy of a black hole by counting up the number of string states in this system. Also, Strominger and his group have discovered that strings and quantum black holes are tied together via a phase transition that transforms free quantum strings into quantum black holes. The net result of all of this is that there is now an alternate way of describing black holes that is quantum mechanically correct when compared to the classical 'Einstein' solutions for these objects. There is even an explanation for where information goes when it falls into a black hole!