Gameloft's BOGO deal on HD games for the HTC EVO is actually still going on, but if the title you want is Hero of Sparta HD and you don't want to have to buy anything else to get it free, then head on over to gameloft.com from your EVO or gameloft.com/free-hd-game from your computer right now.
Like it did a few months ago with Let's Golf, Gameloft is giving away a game for free if you download and open its HD games bookmark. The game up for grabs right now at no cost to you is—you guessed it—Hero of Sparta HD.
The bookmark can be promptly uninstalled after it's opened and the game will still run just fine, though because it's Gameloft we're dealing with here, you won't be able to transfer the downloaded game to a new device or restore it to your EVO if you hard reset or flash a new ROM. The company does allow you to re-download games you bought (on the same device with the same number) within 90 days of the original purchase, but since you're getting Hero of Sparta HD for free, it doesn't fall under that policy.