A no nonsense, friendly yet informative etymology site. The Theory page has a great overview of etymology and where words come from. Also check out the archive and the lists of roots and prefixes.
Julia Hirschberg is a specialist in computational linguistics. Three of her course outlines are available online, along with dozens of PowerPoints such as Lexical Semantics (rather technical in places but includes useful slides on polysemy, parsing and much more). Also check out the research projects section
The Tongue Tied Untied (Guide to grammar, punctuation and style) is an online book which includes printable exercises so that you can check your learning as you progress. Thanks to Ruth Wood at Pro-Training, Colwick, Nottingham who recommends this link for text analysis assignments.
Ideal for reference. Sadly the useful online tests to check learning (such as the challenging mastery grammar quiz) are no longer available for free but the free punctuation, capitalisation and grammar rules, and commonly confused words are all excellent.
Apostrophe protection society not only gives you rules for using apostrophes but also provides dozens of real-life pictures showing examples of misuse. Great for teaching.