iftop - iftop does for network usage what top(1) does for CPU usage.
Vistumbler - Vistumbler is a wireless network scanner for Windows Vista and Windows 7. Features:
vnStat - vnStat is a console-based network traffic monitor for Linux and BSD that keeps a log of network...
inSSIDer - inSSIDer Essential makes it easy to maintain small business WiFi.
nload - Monitor network traffic and bandwidth usage in real time
Homedale - With Homedale you can search for Wi-Fi / WLAN Access Points and monitor their signal strength.