Most academic research today is at least somewhat interdisciplinary. For historians, theories and methods are sometimes borrowed from other disciplines such as philosophy, literary criticism, and anthropology.
Furthermore, the publications compiled in the service of other disciplines very often serve as valuable source material for historians.
Check these links for helpful introductions to research resources for related disciplines:
African American Research Center
Online Resources for Religious Studies
Philosophy Library Resource Guide
Social Sciences, Health, and Education Library (SSHEL):
International and Area Studies Library
Literatures and Languages Library (includes Classics Collection)
Music and Performing Arts Library