Meanwhile, sequel Halo 4 is currently on sale for $10 at Best Buy through the end of the week.
Might & Magic: Clash of Heroes was free on Xbox Live October 1-15.
The free games are part of Microsoft's new Games With Gold program that gives Xbox Live Gold subscribers two free titles each month until December 31. Users can keep the games forever.
For more on Halo 3, check out GameSpot's review.