Quill is an excellent new site for working on proof reading skills such as punctuation and grammar that I just learned about from Richard Byrne's blog. This site allows students to login and correct Reading passages that teachers have selected. Best of all, Quill analyzes the results and shows students what they have gotten wrong w/ a walkthrough explaining the correct answer.
No Red Ink is a fantastic free site for students to help improve their writing and grammar skills. NRI uses differentiated learning so it adapts to student's needs and focuses on the areas they need help in. Also, NRI focuses on a student's content area interest to keep writing interesting and exciting. Finally, teachers can use NRI for testing and assessing a student's needs as well as generate maps/reports for student/class tracking.
The Grading Game is a wonderful iOS app that I just found out about from the equally wonderful site, Edshelf. This is a fun and sometimes hilarious game for finding typing and grammar mistakes. It's a very unique and easy to use game where people race against time to find all the mistakes. Also, it is a good way to reinforce grammar and helps students strengthen their skills.
GrammarBase is a nice online tool for checking grammar, spelling, plagiarism, and more. All a user has to do is copy/paste text and then click check. Then for a fee GB will fix all the problems.