My Take
Learnzillion is an incredible resource of "how to" for typical classroom activities. Especially focuses on reading and writing, with some math. Like the name says, there are zillions of things here. Heavy on quantity, but it does have a wide range of quality. Need to write and outline, you can find that on Learnzillion. Need to write a descriptive paragraph, you find that on Learnzillion. Need to write an expository essay, you can find that on Learnzillion. You can find out how to write almost everything on Learnzillion.
How I Use It
I send students to Learnzillion to brush-up on techniques that they lack sufficient preparation for. It's a great resource for specific "how to"s. For instance if students need a reminder of how to write a 500 word expository essay on their chosen topic, this the place to go. I only use it this way, but you could use it for helping students write better, helping students understand the structure of an essay, and how to collect and organize information fro many types of writing.