Common Sense Media's website has a number of links to writing applications and websites:

 

According to their site, "These apps and websites cover several of the skills that kids need to become strong writers. Whether you have a preschooler just learning to form letters or an older kid who's learning the finer points of constructing stories and self-expression through writing, you'll find excellent learning tools on our list." 

Here's a link to Level it Books, a really useful application that can be installed on to iPhones, iPads, and iPod touches:

The application gives you the ability to scan a book's ISBN # to view the Lexile, Guided Reading, Grade Level Equivalent, and/or DRA levels for the book.

The following site has a number of links on the following topics: