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
www.brown.edu/Student_Services/Writing_Center/

Harvard University
http://writingcenter.fas.harvard.edu/

Purdue University
owl.english.purdue.edu

Yale University
http://writing.yalecollege.yale.edu/

UNC
Great selection of handouts that answer almost any grammar or writing question you have http://www.unc.edu/depts/wcweb/handouts/index.html

Small Colleges

Amherst College

Some good suggestions under the "Writing Advice" heading and a great guide for writing with sources.
www.amherst.edu/~writing/

Swarthmore College
http://www.swarthmore.edu/writing/writing-center