Guide to Grammar and Writing
While this is an excellent source for grammar, it also has a number of topics related to paragraphs, essays, and research papers. The layout offers pull-down menus by category, and it also has a number of PowerPoints and exercises.
Grammar Girl provides short, friendly tips to improve your writing via a weekly podcast. Covering the grammar rules and word choice guidelines that can confound even the best writers, Grammar Girl makes complex grammar questions simple with memory tricks to help you recall and apply those troublesome grammar rules.