Cole Porter:

The Cole Porter Reference Guide

Includes production information;
cross references all of Porter’s songs, shows and movies.

Broadway history:

Musicals 101

An encyclopedic resource about Broadway and musicals in America

P.G. Wodehouse:

The Wodehouse Society

The U.S. society website offers links to many other sites

Musical Theatre:

The Guide to Musical Theatre:

Includes information about London’s West End as well as Broadway.