The Purdue OWL has a wealth of information on the writing process, the research process, grammar, citing sources, style guides, job search writing, graduate school applications, subject specific writing, and English as a Second Language.
The two sites below feature the writing of Lynn Gaertner-Johnston, a writing consultant who founded the Syntax Training company.
The Free Management Library (linked to below) has several sections related to Business Communication topics. See the links below for these sections.