M-U-G-S: Mechanics, Usage, Grammar, and Syntax ~ the easy way to make your writing stompier!
Here's a quick description of what MUGS are.
In each of the following subcategories, you'll find online exercises to practice the areas of MUGS that you need to improve upon.
Mechanics
Usage
Grammar
Syntax
Towson University's Online Writing Support site - TONS of resources for MUGS