Outcast - Second Contact is free for a little less than two days at this point
Also, Humble freebies are usually for Steam keys that you have to activate within some time period. Once it's been activated it stays in your Steam account forever.
For this game it looks like there's no Steam key, it's a DRM-free download that you have to download before November 29th. After that point the download link is inactivated and you're responsible for keeping/backing-up the game installer. At least that's how I read the fine print.
So just keep in mind that this is a little different than other Humble free games and you'll be OK.