The best open source alternative to CloudShark is Wireshark, which is both free and Open Source. If that doesn't suit you, our users have ranked more than 10 alternatives to CloudShark and three of them is open source so hopefully you can find a suitable replacement. Other interesting open source alternatives to CloudShark are NetworkMiner (Free, Open Source) and PCAPdroid (Free, Open Source).
Wireshark is the world's foremost network protocol analyzer. It lets you capture and interactively browse the traffic running on a computer network. It is the de facto (and often de jure) standard across many industries and educational institutions.
NetworkMiner is a Network Forensic Analysis Tool (NFAT) for Windows. NetworkMiner can parse PCAP files and regenerate/reassemble transmitted files and certificates from PCAP files with HTTP, FTP, SMB, SMB2, TFTP and several other protocols.
PCAPdroid is an android app to capture the phone traffic and analyze it remotely (e.g. via Wireshark). The traffic can be easily downloaded from a remote device thanks to the integrated HTTP server, or streamed to a remote UDP receiver.