The Internet provides a number of venues where one can polish their grammar skills or review basic components. One of the most thorough and extensive is the Online Writing Lab (OWL) provided through Purdue University. Another is Dr. Grammar, a multifaceted resource offered by the University of Northern Iowa.
Purdue's OWL contains grammar-related help for a wide range of topics, including but not limited to revision of spelling, articles, pronouns, prepositions, as well as the use of adjectives and adverbs. The site is especially geared to aid students or other writers engaged in research projects. Hence the mechanics of citation are also covered.
Dr. Grammar has many features in common with OWL, along with some others. Dr. Grammar further supplies special sections pertaining to those writing English as a second language, business-oriented communication and another section devoted to etymology, or word origins.
Another powerful tool is Grammar Girl, a site created by experienced writer and editor, Mignon Fogerty. Articles on Grammar Girl are intended to be detailed, as well as fun and accessible. In this sense, the reader is perhaps likely to find these articles less academic sounding than other sites. For those wishing to brush up on grammar using books rather than the websites, two of the most canonical sources are Strunk and White's "The Elements of Style," and William Zinsser's "On Writing Well." Both of these texts can be readily found either in hard copy, or in PDF form online. The strength of these particular texts is their ability to blend grammar exercise and revision with the fundamentals of clear, effective writing craft.