Here's a few basic links for you:

A short synopsis of the history of pietism
An Article showing how pietism influenced philosophy and liberal theology
The Archive of the Pietism Studies Group

Herrnhut was the Moravian Brethren colony established by Count Nicholas von Zinzendorf. It was the Moravians who were responsible for the conversion of John Wesley. Thus, Moravians are also responsible for the Arminian theological strain within the English-speaking evangelical world. Incidently, the Moravians were also pietists.