The Visitor Stats plugin has this feature. In fact, it shows you all users who logged in for a given day. You can see a working example of it
here. You can download the plugin
here.
Another option is a block I created that shows your current session information. I was having issues with remote IP addresses so I created this block to help debug. You can find the session block
here.
In a world without walls and fences, who needs Windows and Gates?