I make enough of them myself, and get asked often, so I figured it was time to post some of my reference materials for basic English usage, organized by the most common types of mistakes.  I have hundreds, and will cull through them and update this frequently. This is merely a start.  If you have great tutorials or guides, please post as a comment.

Apostrophes

• How to use Apostrophes (Interactive Tool– GREAT)

Capitalization After A Colon?

• On Capitalizing After A Colon

Commas — Before Quotation Marks

• Major Comma Uses (towson.edu)
• Before Quotation Marks (eHow Tutorial)

Hyphens

• Using Hyphens (Purdue)

I vs Me

Between you and me, this is one of the biggest of the errors seen on the web after "looser" used for "loser." (See Looser)

• I  vs. Me
• Between You And Me
• I/Me/Myself

Looser vs Loser

• Looser means something that is not firmly held or fixed in place.
• Loser is what you appear to be when keep confusing Looser and Loser and calling it a typo when anyone points it out.

Your vs You're

• How to use You're and Your

General Guides & Tutorials

• Purdue OnlineWriting Lab (OWL) — Comprehensive
  
• Common Errors in English Usage
• Jack Lynch's Guide to Grammar and Style (Personal Favorite Site)
  

Related Guides and Tutorials

   Argument

• A List Of Fallacious Arguments
  
• Fallacy of False-equivalence
• Generalized definition of 'false equivalence'.
• Propaganda Techniques

  Grammar Myths & Myth Busters

• Grammar Myths (Grammarphobia.com) — Patricia T. O'Conner and Stewart Kellerman met as editors at the New York Times. By the time their collaboration began, they had more than half a century of experience as writers and editors between them.

   Web & Document Design and Typography

• Designing Documents With A Word Processor
• Most Used and Abused Web Design Trends of All Time

   Plagiarism and Ethics

• Plagiarism (WIkipedia) — "Plagiarism is not only the mere copying of text, but also the presentation of another's ideas as one's own, regardless of the specific words or constructs used to express that idea. In contrast, many so-called plagiarism detection services can only detect blatant word-for-word copies of text."
• Avoiding Plagiarism (Tutorial)

   Vocabulary

• Common Word Traps
  
• Vocabulary Workshop (American University

   Dictionaries

• Google Dictionary — Finally—and years late—a dictionary that pulls multiple meanings from multiple sources without the annoying ads or interface drama of other meta search engines. Be sure to get the plug-ins for your browsers. I have one for my Ubuntu linux desktop. I can double click any word, in any application or tool, and then click one desktop button to launch a browser with the Google Dic. results.  A fantastic resource that should have been possible in the 1980s. We had the technology.