Great tool for grammar "mini-lessons" to address specific issues in student writing.
The Grammar Girl site is a great resource when you want to address an issue in the context of student writing through a mini-lesson.
How I Use It
When a grammar issue emerges in student writing--i.e. I notice a large number of students are having issues with comma splices, I can take a moment to offer a mini-lesson on comma use. The Grammar Girl site, especially the videos, is a great resource for these lessons. Students like her hip writing style and pay attention to Grammar Girl's advice. The site is also great for independent learning; I can send students to it who are having a particular problem so they can work on it on their own.