Wi-Fi PCAP Capture uses an external USB RTL8187 Wi-Fi card and does not require a custom ROM or root privileges.
You will need a device running Android 4 or newer (for USB host support), a working USB host mode implementation (some manufacturers do not have a working implementation), a USB OTG cable or USB host port, and a RTL8187 based wireless NIC, such as the Alfa One.
Currently other wireless chipsets are not supported, though hopefully in the future they will be. Capture from the internal wireless device is not possible at this time.
Wi-Fi PCAP Capture is GPL, source is available on the website at http://www.kismetwireless.net/android-pcap/