Graphion's Online Type Museum http://www.slip.net/~graphion/style.html
Good advice about indenting, using capitals, spacing, and leading. Hyperlinked definitions and examples round out the presentation.
A Glossary of Typographic Terms http://www.slip.net/~graphion/gloss.html
A good starting point for anyone interested in design and layout in print and online venues.
A Visual Thesaurus http://www.plumbdesign.com/thesaurus/
A strange and captivating site that gives a visual representation of synonyms; requires high-end computer with Java capability.
Stats Every Writer Should Know http://nilesonline.com/stats/
This site describes writers' need for understanding and interpreting statistical data.
Ten Common Typesetting Mistakes http://vera.inforamp.net/~poynton/notes/typesetting/
This explanation of common typesetting mistakes made by those using word processors gives a good thumbnail guide to document planning and desktop publishing.