Correct Punctuation
A brief guide to correct English punctuation
Grammar Bytes
Gives grammar terms definitions and grammar rules, contains a lot of interactive exercises. The website can also be helpful for English language teachers because it contains handouts for classes.
Grammar Help by Ruth Vilmi
A very good source for grammar practicing; gives explanations about most areas of grammar and some issues on punctuation. Also contains a great number of quizzes on many topics and of different complexity. Has also links to other websites on grammar.
Grammar Slammer
Gives explanations and examples of a number of grammar and punctuation issues
Guide to Punctuation by Larry Trask at the University of Sussex
Gives very detailed explanations on practically all areas of punctuation
Index to the Guide to Grammar and Writing: Capital Community College Foundation
A very useful site for grammar problems connected with advanced writing, including punctuation, sequence of tenses, compound nouns and lots more
Punctuation ay HyperGrammar at the University of Ottawa
A detailed look at how to use punctuation effectively and correctly; discusses each mark in turn, with a quiz in each section to test understanding
The Blue Book of Grammar and Punctuation by Jane Straus
Gives rules for a number of grammar and punctuation issues, some of them quite rare (e.g., rules for writing numbers) and gives an opportunity to practice skills in a quiz.