Chomp chomp Grammar Bytes

Some of the grammar sites on the Internet can be quite dry and difficult to read, but Grammar Bytes is a refreshing change. It is self-professed "grammar instruction with attitude".
The site features interactive exercises as well as explanations of grammar terms. It is well laid-out, easy to navigate and the text is clear and easy to read. It's useful for once-off grammar checking, polishing up your rusty grammar, or becoming that apostrophe-obsessed grammar grandma you've always wanted to be ;)