Per Lifehacker:

According to the Daily Dot, nearly 5 million usernames and passwords associated with Gmail accounts have been leaked on a Russian Bitcoin forum. Here's what you should know.

The list has since been taken down, and there's no evidence that Gmail itself was hacked—just that these passwords have been leaked. Most sources are saying that lots of the information is quite old, so chances are they were leaked long ago—though others are claiming 60% of the passwords are still valid (not to mention really, really horrible).

Now would be a good time to change your password(s) and set up two-factor authentication.