An Overview of Who Logged In
The Who Logged In module creates a block that lists users who logged into the site to a certain time period. You can configure the module to any time you like but by default it is set to 24 hours.
The Who Is Online core block that ships with Drupal is similar but the difference is that the Who Is Online block only displays users who are currently logged in but the Who Logged In block also displays users who logged in from a certain time period. (For example last 48 hours).