As it currently stands, you must have Gold privileges - either from your account, as part of a Family Pack, or as part of a promotion - to play online.
Free (formerly known as Silver) and local accounts cannot otherwise play online.
At one point, Microsoft did offer online play to Free/Silver members for select titles , but it was for a limited time, and that no longer appears to be the case.
Final Fantasy XI is an exception, though: you can play online even with an Xbox Live Free account .
gaming.stackexchange.com/users/8366/Dave DuPlantis
As far as I am aware, it is only possible to play multiplayer games online through Xbox Live if you have a gold account, or if you are playing on a Free Gold Weekend, which are special, and often rare, events that happen often to celebrate the release of a new title or occasionally for holidays.
gaming.stackexchange.com/users/9992/Edward Black
You can play online with silver on certain games if you're on the same console and playing with a person who has gold.
For example, the Halo games, although various limitations apply.
There is a somewhat relevant list here.
gaming.stackexchange.com/users/13215/SirYakalot
You can play some games, as I have Crackdown 2 and I can play on multiplayer still.
gaming.stackexchange.com/users/29279/Jesus
I just went on cod mw3, for some reason it's letting me play on silver.
I don't have any complaints!
gaming.stackexchange.com/users/30957/keith
I do not own an XBox, but a friend of mine does. I recently changed my regular on-console gamer profile to an XBox Live Silver profile so that I can use my profile on any XBox. I do want to play online occasionally, but I have heard that it is not possible to play online with a Silver profile.  I've seen several places say that its possible to play with online for select games; is this true? Where might I find such a list?  Thanks!
Is it possible to play online with an XBox Live Silver account?
gaming.stackexchange.com
<xbox-live><xbox-360>
gaming.stackexchange.com/questions/24257
gaming.stackexchange.com/users/10264/NT3RP
You can only play plutiplayer games if you have a gold account, or if you have a silver account and are playing on the same console as someone with a gold account.  There is a short list of games available online with an XBox Live Silver account, mainly with some restrictions.
There seems to be a short list of games available online with an XBox Live Silver account.