Check out Learn Learn American English Online, a multi-level grammar course with interactive exercises that give you immediate feedback. The English Page is another good resource.
The Longman English Dictionary Online is a recommended dictionary as well as the Oxford English Dictionary.
The Visual Dictionary Online and Visual Thesaurus are useful dictionary tools as well.
The NTID ASL Video Dictionary is available free to the NTID community.
There are photo and picture/visual dictionaries available which are helpful for looking at vocabulary categories.
Gallaudet University Writing Center Take a look at Essays, How to Avoid Plagiarism, and Words that Introduce Quotes and Paraphrases--use these different verbs for your essays-use 'strong' verbs for argument essays
Capital Community College Foundation Writing Resources
Check out Linda Blanton multi-level composition books with essay examples. Each essay focuses on one grammar concept at a time.