These sites are packed with information and suggestions for writers in every discipline. Some also include an in-depth section on cover letters, resumes, grad school essays and grant proposals.

Large Colleges 

Brown University

Harvard University

Purdue University

Yale University

Great selection of handouts that answer almost any grammar or writing question you have

Small Colleges

Amherst College

Some good suggestions under the "Writing Advice" heading and a great guide for writing with sources.

Swarthmore College