Monitor Network Traffic on WPA

Solution 1:

WireShark can do this. You can even tell WireShark your WPA key so it can decrypt it for you:

More in How to Decrypt 802.11.

Solution 2:

AirView is a wireless packet monitor for the 802.11b Wi-Fi and Ethernet wired LAN networks packet sniffer. AirView captures wireless network packets, and helps you to visually monitor the WLAN packet activities. Airview 1.0 for WiFi captures every packet on the air and supports both Ethernet LAN and 802.11 and displays network traffic at the MAC level. You see real 802.11 frames, not just the data after it has been filtered through an Ethernet emulation.

AirView is open source freeware and portable.

Kismet is an 802.11 layer2 wireless network detector, sniffer, and intrusion detection system. Kismet will work with any wireless card which supports raw monitoring (rfmon) mode, and can sniff 802.11b, 802.11a, and 802.11g traffic.

Kismet identifies networks by passively collecting packets and detecting standard named networks, detecting (and given time, decloaking) hidden networks, and infering the presence of nonbeaconing networks via data traffic.

Kismet is open source freeware (GPL).