Remember when this happened with
Fable 2? Well, It's happened again.
Fable 3 is currently listed as "Free" on the Xbox Live Marketplace. I can only assume this is a glitch in their system or a hint that
Fable 4 may be announced at the Press Conference in a few hours. Either way, better grab it just in case it disappears soon.
Go to Games -> Browse Games -> On Demand -> A-F -> F -> Fable 3.
Not sure if this can be done via their website, but for the time being, you can do so through your Xbox Live Account and you must be a gold member for this to work.