Is it possible to play online with an XBox Live Silver account?

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.

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.

You can play some games, as I have Crackdown 2 and I can play on multiplayer still.

I just went on cod mw3, for some reason it's letting me play on silver. I don't have any complaints!

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.