I found this post on Smashing Magazine: "50 Free Resources That Help Improve Your Writing Skills" and I am posting it here in seven segments. This is the first segment:

Grammar & Punctuation

Use English Punctuation Correctly
A quick and useful crash course in English punctuation.

HyperGrammar
An extensive electronic grammar course at the University of Ottawa’s Writing Centre.

Grammar Girl
Mignon Fogarty’s quick and dirty tips for better writing. Grammar Girl provides short, friendly tips to improve your writing. Covering the grammar rules and word choice guidelines that can confound even the best writers, Grammar Girl makes complex grammar questions simple with memory tricks to help you recall and apply those troublesome grammar rules.

Better Writing Skills
This site contains 26 short articles with writing tips about ampersands, punctuation, character spacing, apostrophes, semicolons and commas, difference between i.e. and e.g. etc.

The Guide to Grammar and Writing
An older, yet very useful site that will help you to improve your writing on word & sentence level, paragraph level and also essay & research paper level.

Writer’s Block
A compact resource with over 20 articles that cover abbreviations, capitalization, numbers, punctuation, word usage and writing styles.

Paradigm Online Writing Assistant
This site contains some useful articles that explain common grammar mistakes, basic punctuation, basic sentence concepts etc. Worth visiting and reading. The Learning Centre contains similar articles, but with more examples.

Jack Lynch’s Guide to Grammar and Style
These notes are a miscellany of grammatical rules and explanations, comments on style, and suggestions on usage put by Jack Lynch, an Associate Professor in the English department of the Newark campus of Rutgers University, for his classes.

English Style Guide
This guide is based on the style book which is given to all journalists at The Economist. The site contains various hints on how to use metaphors, punctuation, figures, hyphens etc. Brief and precise.

Technical Writing
An extensive guidance on grammar and style for technical writing.

40+ Tips to Improve your Grammar and Punctuation
“Purdue University maintains an online writing lab and I spent some time digging through it. Originally the goal was to grab some good tips that would help me out at work and on this site, but there is simply too much not to share.”