For the next 15 hours or so (from the time of posting, at 8:45 PM Central Time), the Humble Bundle is offering Tropico 3 for free on Steam. Just follow this HUMBLE BUNDLE LINK and fill out the form, connect to your Steam account, activate and download your game. The Tropico series is a construction and resource simulator (as well as a political sim) with an island dictator theme.
Several other games are on sale at the moment to, and as always there are multiple bundles available at various prices, which can you can dictate how much goes to charity, the Humble Bundle team, and the developers of the games via the pay what you want system. My Steam Library is full of backlogged games in no small part due to the awesome deals presented via the Humble Bundle.