St. Cloud State University and LEO: Literacy Education Online
http://leo.stcloudstate.edu
Online Resources for Writers
http://wwwold.ccc.commnet.edu/writing/writing.htm
Purdue Online Writing Lab (OWL)
http://owl.english.purdue.edu/
Del Mar College -- Stone Writing Center
http://www.delmar.edu/engl/wrtctr/composition.html
Garbl's Writing Center
http://wwwold.ccc.commnet.edu/grammar
An excellent site with lots of different categories to explore. Some of the categories include action writing, plain language, style and usage, to name a few. You will also find an exhaustive list of grammar guides.