The Network Activity Indicator is a free tool that will restore the old Windows XP "two monitors" system tray icon to Windows 7.
If you don't remember the icon, it flashed to show network activity, which was often useful. But, unfortunately, it disappeared in Windows 7; apparently the designers wanted to make it "less noisy".
Network Activity Indicator doesn't only bring the icon back, though - it also allows you to configure how it works. So you can have the icon flash to indicate all traffic, just specific packet types (TCP, UDP or ICMP), or even a specific network interface.
And despite all this extra power, Network Activity Indicator uses the absolute minimum of system resources. The compact executable is only 336KB; it doesn't need any installation; and when running in the background, the program consumes only a little more than 1MB RAM (private working set).