There are many OWL's on the Web. These are among the best.

Purdue University's Online Writing Lab
http://owl.english.purdue.edu/
A comprehensive, clearly organized site. Includes valuable resources for writers and for teachers of writing. See especially the extensive collection of handouts dealing with various writing skills and grammar problems.

Paradigm Online Writing Assistant
http://www.powa.org/
An online guide to writing, Paradigm offers clear instruction about writing informal, thesis, argumentative and exploratory essays. Students can find out how to document sources and edit their work.

Writer's Handbook
http://www.wisc.edu/writing/
This writing center from the University of Wisconsin-Madison is easy to use and several good sections aimed at beginning writers.

University of Richmond's Writer's Web
http://writing.richmond.edu/
This is a basic guide of the writing process. Scroll down the page to access links to information about categories ranging from Writing First Drafts, through Sentence Structure, Using Sources Effectively, and Editing for Clarity & Style. There are good articles about writing papers in general as well as about the more specific aspects of writing skills.