Just about everything on the Live.com domain is down, for us and for our readers around the world.  A ping to live.com times out:

EDIT: Well you learn something new every day: Microsoft does not respond to pings, according to this tweet:

The Live ID service was definitely down, however.

Our Twitter followers are reporting that Zune and Xbox Live users are not able to sign in, either, as they use Live ID.  Live Mesh appears to be up, however, again according to our followers:

OK All Better Now

While writing this post, live.com has come back up.  Microsoft is investigating the outage, according to tweeters.

UPDATE:

@WindowsLive, the official Twitter account of Microsoft Windows Live, acknowledges the issue in a 3 part tweet