Turtle Academy is a free online project which enables users to learn the Logo language and programming principles. Students begin with lessons which involve basic turtle commands, through to controlling the Turtle and pen. Then students can enter the playground and begin to program their own works of art.
Before using this program, I recommend students become with Turtle Art as this program uses a drag and drop block interface (link for previous blog post on Turtle Art). Then, once mastering this, they can move onto Turtle Academy which requires general-purpose programming language.