WinPcap is the industry-standard tool for link-layer network access in Windows environments: it allows applications to capture and transmit network packets bypassing the protocol stack, and has additional useful features, including kernel-level packet filtering, a network statistics engine and support for remote packet capture.

More details: www.winpcap.org

Sniffer of HiDownload Platinum use winpcap's API to watch network traffic and identifying potential urls.