Grammar Bytes! Interactive Grammar Review
Activities review comma splices, fused sentences and verb tense. More topics are being developed.

GRAMBO
This is a short quiz on general sentence structure. Constructive criticism is offered if you answer incorrectly.

The Internet Grammar of English
Click on the Search link in the sidebar. Enter sentence fragment, run-on sentence, etc. in the search field.

English Grammar Connection
Select a subject and a skill level from the drop down menus and then press the Start a Lesson button.

English Grammar Style and Theme Page
 

Web English Teacher – Grammar Resources
Sentence fragments, proof reading, mechanics, practice quizzes, etc.

Relative Clauses – English Grammar Online
Defining and non-defining relative clauses

Owl Online Writing Lab
Grammar, Spelling and Punctuation by Purdue University 

The Learning Network – The Grammar Gorillas
a game to practice parts of speech

Grammar: A Review of the Eight Parts of Speech
 

Traditional Grammar: An Interactive Book
 

Daily Grammar
 

Common Errors in English
The page also links to non-errors, commonly misspelled words, suggestions, and more 

Scroll down the page until you see the phrase “Go to Errors”