http://www.ccc.commnet.edu/grammar/
The Guide to Grammar and Writing contains scores of digital handouts on grammar and English usage, over 170 computer-graded quizzes, recommendations on writing.
 

http://www.edufind.com/english/grammar/index.cfm
Go to this site and click on Table of Contents for a variety of grammar information.
 

http://www.dailygrammar.com/
Daily Grammar is a free service of Word Place, Inc.  Much like A Word A Day (no affiliation), which teaches you a new word each day, Daily Grammar sends you e-mail messages with a grammar lesson five days of the week and a quiz on the sixth day.  You can look at all the lessons in our Archive section.
 

http://englishplus.com/grammar/
Welcome to Grammar Slammer, the help file that goes beyond a grammar checker. Use it as you would any help file.
 

http://www.funbrain.com/grammar/
The Grammar Gorillas gives fun sentences and asks you to find the noun, verb, etc.
 

http://owl.english.purdue.edu/handouts/grammar/
Purdue University - Grammar. Adjectives and adverbs Nouns Prepositions Pronouns Sentence structure Verbs. ... Grammar Spelling Punctuation. Grammar, Punctuation, and Spelling. ...
 

http://a4esl.org/q/h/grammar.html
Online grammar quizzes
 

http://www.chompchomp.com/menu.htm
Grammar Bytes! Grammar explanations, handouts for teachers and students, and interactive exercises.
 

http://aliscot.com/bigdog/
Here you have a list of the bare essentials of grammar. These are the things that English teachers love to comment on in your papers. They really are important, and, no, those profs don't just mark them so they can keep you from ever getting anything higher than a C on a paper. (Spanish version available)
 

http://eslus.com/LESSONS/GRAMMAR/POS/pos.htm
Free video-enhanced course includes example sentences and quizzes