The final touches a writer puts on a piece of work is proofreading for grammar and mechanics. The following sites offer comprehensive guides and practice exercises for grammar and mechanics topics:

Guide to Grammar and Writing
From Dr. Charles Darling, Capital Community College, use the pull-down menus next to each grammar or writing topic to select a topic. The grammar quizzes provide good practice!

Exercise Central
From Bedford/St. Martin's, these interactive grammar quizzes are sorted into multiple levels for practice at your own speed. Free registration is required.

Grammar, Punctuation and Spelling
From Purdue University OWL, these handouts, PowerPoint presentations, and exercises provide help with grammar, spelling and punctuation. It's easy to find what you are looking for here!

For help with the most common grammar mistakes, try the following from Literacy Education Online:

Commas

Sentence Fragments

Run-On Sentences and Comma Splices

Pronouns

Verb Tenses

Subject/Verb Agreement

Active and Passive Voice

Parallelism