The Armchair Grammarian
http://community-2.webtv.net/solis-boo/Grammar1
"...the Armchair Grammarian. Here you will find a compilation of the fundamental usage of English grammar and the underlying rules of punctuation."
Common Errors in English
http://www.wsu.edu/~brians/errors/index.html
A thorough discussion on the rules of English along with an understanding that these rules can be "picky." click the Go to the list of errors link for additional websites. Scroll to the bottom of the screen.
The Blue Book of Grammar and Punctuation
http://grammarbook.com/
An excellent companion website to the book of the same name. Not only has Jane Straus included easy to read rules, but she has included quizzes to test your understanding of these rules. Well worth your time to explore this site. Click the "English Rules" link to get started.
List of Grammar Items
http://www.gsu.edu/~wwwesl/egw/grlists.htm
A great list of sites with information on verbs (regular and irregular), nouns, conjunctions, and even a list of British and American words. So grab a "biscuit" (cookie) and head down the "motorway" (highway) for some fun and interesting differences with your "solicitor" (lawyer).
Subject Verb Agreements
Big Dog's Grammar--http://www.aliscot.com/bigdog/agreement_sv.htm
Making Subjects and Verbs Agree--http://owl.english.purdue.edu/handouts/print/esl/eslsubverb.html
Subject and Verb Agreement--http://leo.stcloudstate.edu/grammar/subverag.html
One of the most important rules of grammar is verbs and subjects must agree in number. You wouldn't say "She are going to the mall." You would say "She is going to the mall." If you are having difficulties with this type of agreement, spend some time at one or all of these pages.
Irregular Verbs in the English Language
http://orwell.ru/library/others/style/e/e_ivbs.htm
This page will help you use the correct form of the infinitive and simple past and past participle.
EnglishClub.com
http://www.englishclub.com/grammar/index.htm
You never thought there was an English club? Well, not in the terms of a club in the sense we know. This page is full of help with parts of speeches and grammar. There are even some quizzes.
English Grammar Lessons
http://www.grammar-monster.com
Great information here! Get help with punctuation, parts of speech, common errors of usage, and much more.
Guide to Grammar and Style
http://andromeda.rutgers.edu/~jlynch/Writing/index.html
Lots of great information on correct words to use, capitalization, and punctuation. Use the letter links at the top to move to a certain category. For example, click the letter "c" to view content that starts with that letter.