Purdue OWL is a great resource to learn how to write the right way. Recommended for Jr High through High School or advanced learners.
Guide to Grammar and Writing is a wonderful site full of information on a great number of topics. I view this site helpful for refreshing the memory or different wording in order to grasp a better undersanding, love the interactive aspects and power points.
Elements of Literature Geared towards 6th - 12th for writing various types of writing.
Great Source Write has lesson plans and more for K-12
Read Write Think has a small selection of writing tools for brain storming, poetry and more. Also be sure to check out the interactive sites as well for 1st - 12th.
Home Education Resources has free printable worksheets for manuscript and cursive.
Super Teacher Worksheets has printable worksheets for both manuscript and cursive.
Houghton Mifflin Harcourt has a nice selection of graphic organizers for brainstorming before writing.
Houghton Mifflin Harcourt also has some shape printables with and without lines for younger writers.
The Homeschool Literary Quartely A place just for homeschool to write and be reviewed.