Following scattered reports on several forums, including the official Xbox forums, it is now rumoured that an Xbox Live Silver Membership can grant you free online play on certain games.
Kotkau reported that one of their readers was able to play Shadowrun for free last week, and since then it has been stated that you can also play online for free on Lost Planet: Colonies and Universe at War.
This is supposedly to do with Microsoft dropping their subscription charges for their Games for Windows Live service, which included cross platform gaming. Whether this is a deliberate move or a knock-on effect is un-clear at this time, and no other games have been named either.
This is not to be confused with the free live weekend that has just finished, which made every game playable for free during that time.
