For something as important as drawing people into your free service (after all, if the service is free then the registered users are the product you are selling) I would have more than just a button with text on it.
Renaud makes a good point about making the button focus on what you want to do. If a user has already found your site / downloaded your app then they probably already know something of what you are providing. Make this button the most important thing on the screen. You should use button size, position, colour, and imagery to really draw users towards clicking it.
You can make the image on the button suggestive of the "free" nature of the service. I can't recommend a particular image though, as this will be related to your branding, but a crude example might be having a big fat arrow with a little sign like this hanging from it.