Extremely comprehensive grammar and writing resource. If you have a question relating to any grammatical concept, the answer can likely be found here. Try the index for specific concepts and the interactive quizzes to sharpen your skills.
The Online English Grammar
Comprehensive commercial site with downloadable resources both free and commercial. Try the subject list, contents, and punctuation pages for specific info.
Online Writing Lab
Great site for in-depth explanations on grammatical concepts and practice exercises.
Easy to use authoritative source on usage and grammatical rules. The blue menu in the center of the page gives easy access to specific rules and examples.
The Elements of Style
Rules of usage and principles of composition.
Great tool for creating properly formatted bibliographic citations. Select MLA format and then select the type of source you are using in your research. If you don't see it, select "more resources" to display additional sources. Carefully follow the directions to produce correct citations.
Excellent grammar resource that includes interactive exercises, handouts, PowerPoints. Have fun!
in English Usage
Easy to use quick reference on common usage problems. Confusing and often misused words and pairs are listed in alphabetical order. Additional informative resources are also available.
Handy, quick-reference for diagramming the various sentence parts that you may encounter.
And More Sentence Diagramming