Facebook

Get it here.
Cost: Free

UDL Guideline 5.2 - Use multiple tools for construction and composition

Chances are, you're already familiar with Facebook.  You may have even already considered its utility for educational purposes.  Whatever those educational purposes are, spelling is likely not one of them.  Facebook provides a spell-checker within the app.

General Critique

Facebook is renowned for being free and easy to use.  I'm sure we all have our complaints about Facebook, but this is not the place for them.  The spell-checker works well.