Dropbox has always embraced simple design with a lot of negative space. Even the graphics on their homepage are subtle and simple.
Thanks to that simple design and negative space, the blue "Sign up for free" call-to-action button stands out from everything else on the page. Since the CTA and the Dropbox logo are the same color, it's easy for the visitor to interpret this CTA as "Sign up for Dropbox." That's one effective call-to-action.
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