Websites on grammar and language are plentiful. Here are some I think are worthwhile, or just fun:
Grammar and editing
Common errors in English usage
Purdue University OWL (Online Writing Lab) (a great resource for students)
Grammar quizzes at Grammar Guide
Headsup: The Blog (news and editing from a journalism professor)
Arrant Pedantry (grammar and linguistics)
You Don’t Say (John McIntyre, editor at the Baltimore Sun)
Lingua Franca (language blog at the Chronicle of Higher Education)
Fun with linguistics
Fritinancy: Names, brands, writing, and the language of commerce
Separated by a common language (British vs. American English)
Sentence First blog
Word-nerdery
Harmless Drudgery (lexicography blog)
Madam Grammar 