Crackdown is currently available for free on the Xbox Marketplace according to the games listing. This is believed to be part of Microsoft's upcoming X018 event, however nothing has been announced as of the time of posting.
At this moment in time its not clear how long Crackdown will be free to download, however as Microsoft don't have the functionality to time limit full game downloads on the Xbox 360, its pretty safe to say you will be able to keep Crackdown indefinitely if you download it now.
Crackdown can be enjoyed on the Xbox 360, Xbox One and even on the Xbox One X in 4K, with the ability to play over system link if you don't have Xbox Live gold. As one of my all time favourite Xbox 360 games Crackdown is certainly worth having in your collection if its not already.
Thanks to Digiex member Gunz4Hire for the heads up.
Link: Crackdown on the Xbox Marketplace