As Spaw says, we have a drivers Licence which is pretty much the same as a universal ID. The SOcial Security card is not really an ID it's just a little card that helps you remember your number. I've NEVER been asked to show it.

Earlier this year though I did have problems getting back into the US from a weekend trip to Canada.

The fact that I was driving a PA registered Car and had a PA drivers license didn't impress the customs official at all. What finally convinced her that I was OK was that I started getting indignant and told her that US citizens didn't need a passport to travel to and from Canada. I guess she thought "This guy is being a bit of an asshole - so he MUST be American"