Websites:

Big Dog’s Grammar
The bare essentials of grammar with examples and exercises.

Charles Darling’s Guide to Grammar and Writing
Well-organized guide to matters from sentence structure to essay structure. Besides taking quizzes on a host of standard grammatical topics, readers can click on button to see a random "notorious confusable" (a pair or trio of words that sound alike, such as "we're" and "where" and "were").

Common Errors in English
List of common errors taken from the book “Common Errors in English” by Paul Brians.

English Grammar Resources
General list of resources to help with the ins and outs of grammar usage. Includes Usage Guides, Common Errors, Thesauri & Dictionaries, Grammar Myths, Style Guides, and resources specifically for ESL.

Grammar Handbook
Explains and illustrates the basic grammatical rules concerning parts of speech, phrases, clauses, sentences and sentence elements, and common problems of usage. From the University of Illinois Urbana-Champaign.

Online English Grammar
Free online reference and guide for English grammar, writing, punctuation and learning with exercises, games and quizzes.

Online Writing Lab
Handouts and exercises on grammar, punctuation and spelling from Purdue University.

Verbix
Great site for everything to do with verb conjugation. The online conjugator conjugates verbs in English and many other languages. Enter any verb and it provides a chart in every possible person and tense. Irregular forms are shown in red. Enter a word in any language in the verb finder and it will tell you if it's a verb, in what language, and will conjugate it for you.