How to find username on remote computer?

I have the IP addresses of some computers on my network. Is there any command line tool like PsExec, which can take the IP Address as input and find out the username of the currently logged in user? I can install stuff on my workstation, but not the others. I can also run Metasploit / NMap / any other such program on my workstation.

For Windows boxes get psLoggedOn. It tells you who is currently signed into the box via console/rdp and who is connected via network shares.

If you don't want to install an app, there is also the commands qwinsta and query session that you can run from the command line like this:

qwinsta /server:name_of_host or query session /server:name_of_host

Try this:

wmic.exe /node:"IP-or-HostName" ComputerSystem Get UserName

Example:

wmic.exe /node:"172.28.1.100" ComputerSystem Get UserName

Output:

UserName
DOMAIN\User

(Yes, /node value must be quoted)