(1) Grammar Girl - I've put a link to her discussion of a common error, the comma splice (even though the link is included under "references" to the right). This refers to the tendency by which two main clauses are separated only by a comma, rather than by a comma and a coordinating conjunction (alternatively, the two clauses can be separated by a period or semicolon).
"Squiggly ran to the forest, Aardvark chased the peeves."
"Squiggly ran to the forest, and Aardvark chased the peeves."
(2) The Sentence Sleuth - This blog is run by a copyeditor who was a guest writer on the Grammar Girl site four or five times. It's a little less polished, but not horrible. And the idea behind it is so simple: take a sentence and point out an error.
Our textbook is obviously a much more broad review of grammar applications, etc. but these sites provide short yet memorable discussion which I think will make the studying process a little more varied and a little easier.