Angry IP Scanner lets you check your network and trace intruders, which will pick up IP addresses and host names of clients who have logged into your network.
SCREENSHOTS
LINUX

 MAC OS X

 WINDOWS

DOWNLOAD
CLICK HERE : 32-bit Executable for Windows 2000/XP/Vista/7 - recommended download
CLICK HERE : 64-bit Executable for 64-bit Java on Windows Vista/7 - if 32-bit version doesn't work.