The Web abounds with sites teaching grammar, punctuation, and spelling. Not surprisingly, most of these sites are provided by educational institutions, teachers, or business-writing consultants. Some are tutorials (masquerading as style guides) for technical communicators. Here are a few sites that I have found useful or that other people have recommended to me.
Purdue University Online Writing Lab has handouts and exercises on grammar, spelling, and punctuation, PowerPoint presentations related to grammar, and an entire section of handouts and resources for learners of English as a Second Language. http://owl.english.purdue.edu
The Tongue Untied: A guide to grammar, punctuation and style http://grammar.uoregon.edu/toc.html
Has exercises and answers, after explanations.
Grammar Bytes! Interactive Grammar Review http://www.chompchomp.com/menu.htm
An appealing site generous with explanations, handouts, interactive exercises.
Grammar, Punctuation, and Capitalization: A Handbook for Technical Writers and Editors (1990). Very thorough and detailed. https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19900017394.pdf (PDF)
Online Technical Writing. A free online course on technical writing by David McMurrey. http://www.prismnet.com/~hcexres/textbook/
Last updated 20 October 2016