Style

William Lutz, author of Doublespeak and other best-selling books on language, shares 39 Steps for Writing Plain English.

"Neutral point of view" and a tutorial aimed at achieving it, from the invaluable Wikipedia.

Basic Prose Style by Craig Waddell of Michigan Tech consists of  sensible, non-technical advice to make your writing clearer and more effective.

Care and Imagination from the SUNY-Geneseo Online Writing Guide.  Unpretentious ideas about important but often overlooked aspects of student writing.  The Geneseo Online Writing Guide as a whole is very useful. 

Grammar, Syntax, Usage, and Mechanics

First stop in this area would be the University of Chicago's set of links, Grammar Resources on the Web, except for the fact that it hasn't been updated in a while and has several dead links.

Jack Lynch's Guide to Grammar and Style:  "The entries here are of two types: specific articles on usage, and more general articles on style. The specific articles cover such mechanical things as when to use a semicolon and what a dangling participle is; the general articles discuss ways to make 'proper' writing even better."

HyperGrammar from the University of Ottawa:  a compact online grammar handbook.  Very useful; American students should, however, avoid some aspects of Canadian spelling advice.

Common Errors in English Usage by Paul Brians of Washington State University has had over 4,000,000 visitors.  It is especially useful for students of English as a Second Language.

The Guide to Grammar and Writing at Capital Community College isn't very pretty, but it is very wide-ranging and useful, discussing specificity, tone, purpose, writer's block, and a great many other issues as well as grammar and usage.  Teachers will likely appreciate the PowerPoint presentations on particular difficulties students have.

Article usage in English can be particularly troublesome for non-native speakers. 

Myths About Good Writing demonstrates that a lot of so-called experts should read more good writers.