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Revenge of the Titans will automatically be added to your Steam Games Library if you’ve activated the Humble Bundle #2 (use the link to get your humble indie bundle download link if you’ve lost it) on your Steam account. You may need to restart your Steam client for it to show up.
There’s a demo and trailer on http://www.puppygames.net/revenge-of-the-titans/ for those that missed out on the Humble Bundle.