The University of Richmond's Writer's Web:  
This site is geared toward the entire process of writing an essay, but it has a very specific and detailed section on grammar use.  The section on commonly misused words seemed particularly interesting.  To navigate, use the headings listed on the wheel image at the top or scroll down the page.  -SNH  10/18/2000

Writer's Workshop Grammar Guide:
(University of Illinois at Urbana-Champaign)
The site was last modified on 28 April 1998, but the information it contains seems highly relevant and useful for students of composition.  The site offers guidelines about Parts of Speech, Phrases, Clauses, Common Usage Problems, and Sentences & Sentence Elements.  -SNH 11/15/2000

Traditional Grammar:  An Interactive Book (Donald E. Hardy):  
This guide appears in a book form, with traditional chapters instead of a simple list, which makes it more complex to navigate.  Once you enter a chapter, there's a brief introduction in one frame and a list of different topics contained within the chapter in another frame.  Hardy also provides quizzes to challenge writers.  Overall, this is a very useful and extensive grammar guide, but its organization may frustrate some who want a quick and easy answer to their questions.  (Northern Illinois University)  -SNH  10/18/2000